Optical fiber cable

ABSTRACT

The present invention relates to an optical fiber cable incorporating a multi-core fiber provided with a plurality of cores and a cladding region. The optical fiber cable has a jacket covering the multi-core fiber. The multi-core fiber is arranged so that a hold wrap holds the cores in a state in which they are provided with a bend of not more than a fixed radius of curvature, in order to reduce crosstalk between the cores.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical fiber cable incorporating amulti-core fiber with a plurality of cores each extending along apredetermined axis.

2. Related Background Art

For realizing large-capacity optical transmission, there are knownmulti-core fibers configured so that a cladding region integrallysurrounds a plurality of cores.

For example, the multi-core fiber described in Document 1 (MasanoriKOSHIBA, et al., “Heterogeneous multi-core fibers: proposal and designprinciple,” IEICE Electronics Express, Vol. 6, No. 2, pp. 98-103, 2009)is able to realize a low crosstalk level because a power transfer ratebetween adjacent cores becomes sufficiently low by changing a differencebetween relative refractive-index differences Δ of adjacent cores to thecladding (the relative refractive-index difference of each core will bereferred to hereinafter as core Δ) by a very small amount (e.g.,0.005%). Document 1 describes that this technique can realize themulti-core fiber with three kinds of cores different in core Δ and withthe cladding diameter of 125 μm. However, nothing is considered aboutbending of the fiber.

SUMMARY OF THE INVENTION

The inventor conducted research on the conventional multi-core fiber andfound the following problem. Namely, the foregoing Document 1 does notassume that the multi-core fiber is in a bending state, as describedabove. For this reason, in the case where the difference of core Δbetween adjacent cores is 0.005% or so, large crosstalk will occurdepending upon the bending state of the multi-core fiber.

The present invention has been accomplished in order to solve theproblem as described above, and it is an object of the present inventionto provide an optical fiber cable with a structure for controllinginter-core crosstalk (crosstalk between cores) in the incorporatedmulti-core fiber at a low level.

In order to solve the aforementioned problem, an optical fiber cableaccording to the present invention comprises a multi-core fiber, and abend providing structure for maintaining the multi-core fiber in a bendstate of not more than a predetermined radius of curvature. Themulti-core fiber comprises a plurality of cores each extending along apredetermined axis, and a cladding region integrally surrounding thesecores.

Specifically, the bend providing structure provides the multi-core fiberwith a bend of the smallest value of radii of curvature R_(th) given byformula (1a) below, where D_(nm) is an intercentral (center-to-center)distance between core n and core m in the multi-core fiber, L_(F) afiber length of the multi-core fiber corresponding to a length betweenrepeater/regenerators in laying the optical fiber cable, β a propagationconstant of each core at a first wavelength, κ a coupling coefficientbetween adjacent cores at the first wavelength, and XT_(S) a maximumvalue permitted as an average of a distribution of crosstalk afterpropagation of light of the first wavelength through the fiber lengthL_(F). Alternatively, the bend providing structure provides themulti-core fiber with a bend of a radius of curvature R given by formula(1b) below, where Λ is an intercentral distance between adjacent coresin the multi-core fiber, L_(F) the fiber length of the multi-core fibercorresponding to the length between repeater/regenerators in laying theoptical fiber cable, β the propagation constant of each core at thefirst wavelength, κ the coupling coefficient between adjacent cores atthe first wavelength, and XT_(S) the maximum value permitted as theaverage of the distribution of crosstalk after propagation of light ofthe first wavelength through the fiber length L_(F).

$\begin{matrix}{R_{th} = {\frac{1}{2}\frac{\beta}{\kappa^{2}}D_{nm}\frac{{XT}_{S}}{L_{F}}}} & ( {1a} ) \\{R \leq {\frac{1}{12}\frac{\beta}{\kappa^{2}}\Lambda \frac{{XT}_{S}}{L_{F}}}} & ( {1b} )\end{matrix}$

Furthermore, the bend providing structure incorporates the multi-corefiber in a helix shape in the optical fiber cable, thereby providing themulti-core fiber with the bend of not more than a fixed radius ofcurvature. In such a bend providing state, the multi-core fiberpreferably satisfies formula (2a) below, where r_(h) is a radius of thehelix, L_(P) a pitch of the helix, and r_(hmin) the smallest r_(h) inthe multi-core fiber. Alternatively, the multi-core fiber preferablysatisfies formula (2b) below, where r_(h) is the radius of the helix,L_(P) the pitch of the helix, and r_(hmin) the smallest r_(h) in themulti-core fiber.

$\begin{matrix}{L_{P} \leq {2\pi \sqrt{{{\frac{1}{2}\frac{\beta}{\kappa^{2}}D_{nm}\frac{{XT}_{S}}{L_{F}}r_{h\; \min}} - r_{h\; \min}^{2}}}}} & ( {2a} ) \\{L_{P} \leq {2\pi \sqrt{{{\frac{1}{12}\frac{\beta}{\kappa^{2}}\Lambda \frac{{XT}_{S}}{L_{F}}r_{h\; \min}} - r_{h\; \min}^{2}}}}} & ( {2b} )\end{matrix}$

In the optical fiber cable according to the present invention, themaximum value XT_(S) permitted as the average of the distribution ofcrosstalk after propagation of the light of the first wavelength throughthe fiber length L_(F)=100 km or more is preferably 0.001. It issufficient that the maximum XT_(S) be 0.001 at a used wavelength, but inconsideration of wavelength multiplexing transmission, it is preferableto assume the first wavelength (used wavelength) to be at least 1565 nmand 1625 nm. The transmission distance is not limited to the fiberlength L_(F)=100 km, either, but it may be, for example, 1000 km or10000 km.

Namely, the maximum value XT_(S) permitted as the average of thedistribution of crosstalk after propagation of the light of thewavelength of 1565 nm through the fiber length L_(F)=1000 km ispreferably 0.001. Furthermore, the maximum XT_(S) permitted as theaverage of the distribution of crosstalk after propagation of the lightof the wavelength of 1565 nm through the fiber length L_(F)=10000 km ispreferably 0.001. The maximum value XT_(s) permitted as the average ofthe distribution of crosstalk after propagation of the light of thewavelength of 1625 nm through the fiber length L_(F)=100 km ispreferably 0.001. The maximum XT_(S) permitted as the average of thedistribution of crosstalk after propagation of the light of thewavelength of 1625 nm through the fiber length L_(F)=1000 km ispreferably 0.001. The maximum XT_(S) permitted as the average of thedistribution of crosstalk after propagation of the light of thewavelength of 1625 nm through the fiber length L_(F)=10000 km ispreferably 0.001.

In the situation in which the bend providing structure incorporates themulti-core fiber in the helix shape in the optical fiber cable, therebyproviding the multi-core fiber with the bend of not more than the fixedradius of curvature, the multi-core fiber preferably satisfies formula(3) below, where r_(h) is the radius of the helix, L_(P) the pitch ofthe helix, r_(hmax) the largest r_(h) in the multi-core fiber, L_(span)(km) a span length, α_(km) (dB/km) a maximum value of transmissionlosses of the respective cores in the multi-core fiber at a secondwavelength, and α_(S) (dB/span) a permissible value per span as a lossincrease due to incorporation of the multi-core fiber in the helix shapein the optical fiber cable.

$\begin{matrix}{L_{P} \geq {\frac{2{\pi\alpha}_{km}L_{span}}{\sqrt{\alpha_{S}( {\alpha_{S} + {2\alpha_{km}L_{span}}} )}}r_{h\; \max}}} & (3)\end{matrix}$

In the present specification a section of the cable betweentransmitters, receivers, or optical amplifiers is called a span and alength of the section is defined as the span length L_(span) (km). Itshould be noted that the first wavelength and the second wavelength donot always have to agree with each other. The reason is as follows: thefirst wavelength means a reference wavelength for defining theinter-core (core-to-core) crosstalk and the second wavelength means areference wavelength for defining the transmission losses.

In the optical fiber cable according to the present invention, thepermissible value per span α_(S) as the loss increase due to theincorporation of the multi-core fiber in the helix shape in the opticalfiber cable is preferably not more than 0.5 dB/span. At the wavelengthof 1550 nm, the permissible value per span α_(S) as the loss increasedue to the incorporation of the multi-core fiber in the helix shape inthe optical fiber cable is preferably not more than 0.3 dB/span. At thewavelength of 1550 nm, the permissible value per span α_(S) as the lossincrease due to the incorporation of the multi-core fiber in the helixshape in the optical fiber cable is preferably not more than 0.1dB/span.

Furthermore, at the wavelength of 1550 nm, a value of the product(α_(km)·L_(span)) of the maximum value α_(km) of transmission losses ofthe respective cores in the multi-core fiber and the span lengthL_(span) is preferably not more than 15.2 and, in greater detail, thevalue of the product (α_(km)·L_(span)) may be not more than 14.4, notmore than 13.6, not more than 12.8, or not more than 12.0, which may beappropriately set according to need. Specifically, the aforementionedformula (3) is defined as in formulae (4) to (8) below under thecondition of the wavelength of 1550 nm.

Namely, when the value of the product (α_(km)·L_(span)) is not more than15.2, the optical fiber cable satisfies formula (4) below at thewavelength of 1550 nm.

$\begin{matrix}{L_{P} \geq {\frac{2{\pi \cdot 15.2}}{\sqrt{\alpha_{S}( {\alpha_{S} + {2 \cdot 15.2}} )}}r_{h\; \max}}} & (4)\end{matrix}$

When the value of the product (α_(km)·L_(span)) is not more than 14.4,the optical fiber cable satisfies formula (5) below at the wavelength of1550 nm.

$\begin{matrix}{L_{P} \geq {\frac{2{\pi \cdot 14.4}}{\sqrt{\alpha_{S}( {\alpha_{S} + {2 \cdot 14.4}} )}}r_{h\; \max}}} & (5)\end{matrix}$

When the value of the product (α_(km)·L_(span)) is not more than 13.6,the optical fiber cable satisfies formula (6) below at the wavelength of1550 nm.

$\begin{matrix}{L_{P} \geq {\frac{2{\pi \cdot 13.6}}{\sqrt{\alpha_{S}( {\alpha_{S} + {2 \cdot 13.6}} )}}r_{h\; \max}}} & (6)\end{matrix}$

When the value of the product (α_(km)·L_(span)) is not more than 12.8,the optical fiber cable satisfies formula (7) below at the wavelength of1550 nm.

$\begin{matrix}{L_{P} \geq {\frac{2{\pi \cdot 12.8}}{\sqrt{\alpha_{S}( {\alpha_{S} + {2 \cdot 12.8}} )}}r_{h\; \max}}} & (7)\end{matrix}$

When the value of the product (α_(km)·L_(span)) is not more than 12.0,the optical fiber cable satisfies formula (8) below at the wavelength of1550 nm.

$\begin{matrix}{L_{P} \geq {\frac{2{\pi \cdot 12.0}}{\sqrt{\alpha_{S}( {\alpha_{S} + {2 \cdot 12.0}} )}}r_{h\; \max}}} & (8)\end{matrix}$

More specifically, in the optical fiber cable according to the presentinvention, the bend providing structure may provide the multi-core fiberwith the bend of the smallest value of radii of curvature R_(th) givenby formula (9) below, where D_(nm) is the intercentral distance betweencore n and core m in the multi-core fiber (which will be referred tohereinafter as a core distance), β_(m) a propagation constant of core m,κ_(nm) a coupling coefficient from core n to core m, and L_(F) the fiberlength of the multi-core fiber corresponding to the length in laying theoptical fiber cable, the formula (9) defining the radii of curvaturewith a probability of 99.99% that crosstalk after propagation throughthe fiber length L_(F) is not more than −30 dB, for all combinations oftwo cores selected from the plurality of cores in the multi-core fiber.

$\begin{matrix}{R_{th} = {\frac{1}{\{ {{erf}^{- 1}(0.9999)} \}^{2}}( \frac{2\pi}{\kappa_{nm}} )^{2}\frac{\pi \; D_{nm}\beta_{m}0.001}{19.09373\; L_{F}}}} & (9)\end{matrix}$

In the optical fiber cable according to the present invention, each ofthe cores in the multi-core fiber preferably has a refractive-indexprofile of an identical structure on a cross section perpendicular tothe predetermined axis.

Furthermore, in order to realize a low bending loss even with the bendof not more than the aforementioned radius, preferably, the coredistance in the multi-core fiber is not less than 40 μm on the crosssection perpendicular to the predetermined axis, and a relativerefractive-index difference Δ of each core to the cladding region is notless than 0.37%.

An arrangement of each of the cores in the multi-core fiber may varyalong a longitudinal direction thereof on the basis of a bending radiusdirection of the bend provided for the multi-core fiber, by means of anelastic twist (twist provided for the optical fiber in a state in whichglass part of the optical fiber is solidified) or by means of a plastictwist (twist provided for the optical fiber in a state in which glasspart of the optical fiber is softened). This configuration means aconfiguration for intentionally providing the effective twist toachievement of low crosstalk. A specific twist amount may be so chosenas to provide the multi-core fiber with the twist of not less than 2π(rad/m) along the longitudinal direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a cross-sectional view and a perspective viewshowing a configuration of an embodiment of the optical fiber cableaccording to the present invention;

FIG. 2 is a perspective view showing a structure example of a multi-corefiber applicable to the optical fiber cable according to the embodimentof the present invention;

FIGS. 3A and 3B are a view showing a cross-sectional structure along theline I-I of the multi-core fiber shown in FIG. 2, and a refractive-indexprofile near each core;

FIGS. 4A and 4B are a table showing equivalent relative refractive-indexdifferences Δ_(eq) being relative refractive-index differences betweenactual refractive indices and equivalent indices, with changes inparameters r and R about bending;

FIGS. 5A and 5B are views showing relations between parameter r andrelative refractive-index difference Δ_(eq) and relations betweenparameter (1/R) and equivalent relative refractive-index differenceΔ_(eq) in the table shown in FIG. 4B;

FIGS. 6A and 6B are views showing effective indices and equivalentindices of effective indices of respective cores in a multi-core fiberwith a bend;

FIG. 7 is a graph showing variation in inter-core crosstalk along thelongitudinal direction of a multi-core fiber with two cores;

FIG. 8 is a graph showing a relation between crosstalk amount χ andcrosstalk variation amount at an initial zero;

FIG. 9 is a view showing a cross-sectional structure of a multi-corefiber with seven cores;

FIG. 10 is a view for explaining a radius r_(h) and a pitch L_(P) of ahelix;

FIG. 11 is a graph showing relations between the radius of curvature Rand the helix pitch L_(P), for plural types of samples with differenthelix radii r_(h);

FIGS. 12A and 12B are views for explaining a twist provided for themulti-core fiber shown in FIG. 2; and

FIG. 13 is a graph showing relations of κ, R_(th), bending loss, andcore diameter against core Δ.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the optical fiber cable according to the presentinvention will be described below in detail with reference to FIGS. 1A,1B, 2, 3A-6B, 7-11, 12A-12B, and 13. In the description of the views thesame elements will be denoted by the same reference signs, withoutredundant description.

First, FIGS. 1A and 1B show a structure of an embodiment of the opticalfiber cable according to the present invention; particularly, FIG. 1A isa cross-sectional view of the optical fiber cable and FIG. 1B aperspective view of the optical fiber cable. FIG. 2 is a perspectiveview showing a structure example of a multi-core fiber applicable to theoptical fiber cable according to the present embodiment (cf. FIGS. 1Aand 1B), and FIGS. 3A and 3B are a view showing a cross-sectionalstructure along the line I-I of the multi-core fiber shown in FIG. 2,and a refractive-index profile near each core.

As shown in FIGS. 1A and 1B, the optical fiber cable 300 of the presentembodiment is provided with a center member 310, a plurality of opticalfibers 100 wound at a predetermined pitch around the center member 310,a hold wrap 250 wrapping over the plurality of optical fibers so as tohold them in a wound state, and a jacket 200 wrapped around the holdwrap 250. Each optical fiber 100 consists of a multi-core fiber 100A,and a resin cladding 130 wholly covering the multi-core fiber 100A. Eachof the optical fibers 100 is wound at a predetermined pitch along thelongitudinal direction thereof around the center member 310, thereby tobe provided with a bend of a fixed radius of curvature. The jacket 200covers the whole hold wrap 250 so as to protect the optical fibers 100from external force. The center member 310 may be a metal material likea high tensile wire, or an anti-contraction material to resistcontraction of the jacket 200. FIG. 1B includes the illustration of onlyone of the optical fibers 100, for simplicity of illustration, but infact all the optical fibers 100 included in the optical fiber cable 300are wound around the center member 310. The optical fiber cable of thepresent invention is not limited to the above-described structure, butit may be, for example, a slot cable wherein a helical slot (groove) isformed in a surface of a cylindrical member, an optical fiber ribbonincorporating multi-core fibers is set in the slot, and the surface ofthe cylindrical member incorporating the optical fiber ribbon in theslot is further covered by a hold wrap or a jacket, so that the fiberscan also be provided with a bend of a radius of curvature of not morethan a fixed value by adjusting the pitch of the helix of the slot.

A multi-core fiber 100A applicable to the optical fiber cable 300 isprovided, as shown in FIGS. 2 and 3A, with a plurality of cores 110A1,100B1-110B3, and 110C1-110C3 (seven cores in the example shown in FIGS.2 and 3A), and a cladding region 120 integrally surrounding these sevencores. In the multi-core fiber 100A shown in FIGS. 2 and 3A, corearrangement is such that the core 110A1 is arranged at the center of across section (plane perpendicular to the predetermined axis AX) andthat the cores 110B1-110B3 and the cores 110C-110C3 are arranged with anintercentral distance (core spacing) of D around the core 110A1.

The cores 110A1, 110B1-110B3, 110C1-110C3 each preferably have arefractive-index profile of an identical structure. Specifically, FIG.3B shows an example of an outline of the refractive-index profile ofeach core in FIG. 3A. In the example shown in FIG. 3B, therefractive-index profile near each of the cores 110A1, 110B1-110B3,110C1-110C3 is a step-index type refractive-index profile (with arelative refractive-index difference Δ of each core to the claddingregion 120).

The below will describe a method for setting an effective index of eachcore in the multi-core fiber 100A.

A power transfer rate F between two cores is represented by formula (10)below.

$\begin{matrix}{{F = \frac{1}{1 + ( \frac{\psi}{\kappa} )^{2}}}{\psi = {( {\beta_{1} - \beta_{2}} )/2}}} & (10)\end{matrix}$

In the above formula, κ is a coupling coefficient between cores andβ_(n) the propagation constant of core n.

A coupling length L (distance where with incidence of light into onecore n the power of the other core m becomes maximum) is represented byformula (11) below.

$\begin{matrix}{L = \frac{\pi}{2\sqrt{\kappa^{2} + \psi^{2}}}} & (11)\end{matrix}$

According to Document 1 cited above, crosstalk can be decreased bydecreasing F or by increasing L, but, in the case of a multi-core fiberhaving the cladding diameter of 125 μm and employing general cores withthe core Δ of 0.4%, it is difficult to set a number of cores in thecladding, by increasing only L to a sufficiently long distance whilekeeping F large.

It is therefore necessary to decrease F. For decreasing F, it isnecessary to increase φ, i.e., to increase the propagation constantdifference between cores, in other words, to increase a difference ofeffective indices between cores. Document 1 above includes investigationon it with simulation. It describes that crosstalk can be satisfactorilyreduced if the core distance D between adjacent cores is not less than30 μm and if the core Δ is different 0.005% between the adjacent cores.For that, Document 1 above proposes the multi-core fiber with sevencores arranged so that the core Δ of each core belongs to one of threekinds of 0.38%, 0.39%, and 0.40% and so that the core distance D betweenadjacent cores is 40 μm.

However, the investigation in Document 1 above involves no considerationto bending of the multi-core fiber. For this reason, a considerablenumber of very large crosstalk cases must be also included in fact,depending upon bending states of the multi-core fiber.

When a multi-core fiber is bent, bending radii of respective cores arevery slightly different depending upon positions in the multi-corefiber. For this reason, optical path differences of the cores are alsodifferent. When the multi-core fiber thus bent is handled as a linearwaveguide, it is necessary to employ an equivalent index as a refractiveindex based on an optical path difference. The equivalent index can beobtained by multiplying an actual refractive index by (1+r/R), asdescribed in Document 2 (Tetsuro Yabu “Hikaridouharo Kaiseki Nyuumon,”pp. 58-63, Morikita Publishing Co., Ltd., 2007). In the abovedefinition, R is the radius of curvature of a core as a reference(reference core), and r a radial departure from the reference core in abending radius direction (cf. FIG. 4A). Any core may be defined as areference. When an actual refractive index of a bent multi-core fiber isn₀(r) and an equivalent index based on the conversion to a linearwaveguide is n₁(r), an equivalent relative refractive-index differenceΔ_(eq), which is a relative refractive-index difference between theactual refractive index and the equivalent index, is represented byformula (12) below, using parameter r and parameter R.

$\begin{matrix}\begin{matrix}{\Delta_{eq} = \frac{{n_{1}^{2}(r)} - {n_{0}^{2}(r)}}{2\; {n_{1}^{2}(r)}}} \\{= \frac{{{n_{0}^{2}(r)}( {1 + \frac{r}{R}} )^{2}} - {n_{0}^{2}(r)}}{2\; {n_{0}^{2}(r)}( {1 + \frac{r}{R}} )^{2}}} \\{= \frac{( {1 + \frac{r}{R}} )^{2} - 1}{2( {1 + \frac{r}{R}} )^{2}}} \\{= \frac{{2\frac{r}{R}} + ( \frac{r}{R} )^{2}}{2( {1 + \frac{r}{R}} )^{2}}}\end{matrix} & (12)\end{matrix}$

FIG. 4B is a table showing the equivalent relative refractive-indexdifferences Δ_(eq) derived from formula (12) above with changes inparameter r and parameter R about bending. In the descriptionhereinafter, unless otherwise noted, the center core 110A1 shown inFIGS. 1A, 1B, and 2 is considered as a reference core. FIG. 5A showsrelations between parameter r and equivalent relative refractive-indexdifference Δ_(eq) in the table of FIG. 4B, and FIG. 5B relations betweenparameter (1/R) and equivalent relative refractive-index differenceΔ_(eq) in the table.

In FIG. 5A, graph G511 shows the relation between parameter r and Δ_(eq)at R=140 mm, graph G512 the relation between parameter r and Δ_(eq) atR=60 mm, graph G513 the relation between parameter r and Δ_(eq) at R=30mm, and graph G514 the relation between parameter r and Δ_(eq) at R=10mm. In FIG. 5B, graph G521 shows the relation between parameter (1/R)and Δ_(eq) at parameter r=40 μm, graph G522 the relation betweenparameter (1/R) and Δ_(eq) at parameter r=30 μM, graph G523 the relationbetween parameter (1/R) and Δ_(eq) at parameter r=20 μm, graph G524 therelation between parameter (1/R) and Δ_(eq) at parameter r=10 μm, graphG525 the relation between parameter (1/R) and Δ_(eq) at parameter r=0μm, graph G526 the relation between parameter (1/R) and Δ_(eq) atparameter r=−10 μm, graph G527 the relation between parameter (1/R) andΔ_(eq) at parameter r=−20 μm, graph G528 the relation between parameter(1/R) and Δ_(eq) at parameter r=−30 μm, and graph G529 the relationbetween parameter (1/R) and Δ_(eq) at parameter r=−40 μm.

With parameter r=40 μm, Δ_(eq) exceeds ±0.02% even if parameter R=140mm. In the case of the multi-core fiber including the seven corescomposed of three kinds of cores with the relative refractive-indexdifference Δ of one of 0.38%, 0.39%, and 0.40% and arranged so that thecore distance D between adjacent cores is 40 μm, as proposed in Document1 above, since the differences of core Δ between nonidentical cores are0.01%, relative refractive-index differences Δ_(eff) between effectiveindices are not more than 0.01%. It is seen from the above discussionthat in the multi-core fiber of above Document 1, Δ_(eq) comes toovertake Δ_(eff) even with just a bend of parameter R=140 mm. Namely, itis understood that in the case of the multi-core fiber of above Document1, even a slight bend makes very small the absolute values of therelative refractive-index differences between equivalent indices ofeffective indices of nonidentical cores, whereby crosstalk between corescan become significant.

When it is considered that a multi-core fiber is wound around a bobbin,the multi-core fiber necessarily rotates because of variation duringmanufacture or variation during winding, resulting in rotation of corearrangement in the longitudinal direction. In this case, even if thecore distance D from the reference core to each core is constant in thelongitudinal direction, the foregoing parameter r varies within therange of the core distance D, depending upon positions along thelongitudinal direction of the multi-core fiber, and locations where thedifference of equivalent relative refractive indices between effectiveindices of nonidentical cores becomes small come to be distributed alongthe longitudinal direction of the multi-core fiber. Such a state isshown in FIGS. 6A and 6B. It is, however, noted that FIG. 6B showsvariation in equivalent indices in a setting where core positions in thecircumferential direction rotate in a certain period in the longitudinaldirection, in a state in which the cores are uniformly bent in thelongitudinal direction and in which the positions of the cores in theoptical fiber are arranged at equal intervals in the circumferentialdirection in the cross section of the optical fiber.

FIGS. 6A and 6B are views showing the effective indices and theequivalent indices of the effective indices of the respective cores inthe multi-core fiber with a bend, which is an example of the effectiveindices converted to the equivalent indices in the case where themulti-core fiber is bent in the same manner as in a state in which it iswound around a bobbin. Particularly, FIGS. 6A and 6B show the effectiveindices and the equivalent indices of the effective indices of therespective cores in the multi-core fiber 100A shown in FIGS. 1A, 1B, and2. FIG. 6A shows relations between longitudinal position of themulti-core fiber and effective index of each core, in which graph G611shows the effective index of the center core (reference core) 110A1located on the optical axis AX of the multi-core fiber 100A, graph G612the effective index of the cores 110B1-110B3 located around thereference core 110A1, and graph G613 the effective index of the cores110C1-110C3 located around the reference core 110A1. FIG. 6B shows thelongitudinal position of the multi-core fiber versus equivalent index ofeffective index in each core, in which graph G621 shows the equivalentindex of the effective index of the reference core 110A1, graph G622 theequivalent index of the effective index of the core 110B1 located aroundthe reference core 110A1, graph G623 the equivalent index of theeffective index of the core 110B2 located around the reference core110A1, graph G624 the equivalent index of the effective index of thecore 110B3 located around the reference core 110A1, graph G625 theequivalent index of the effective index of the core 110C1 located aroundthe reference core 110A1, graph G626 the equivalent index of theeffective index of the core 110C2 located around the reference core110A1, and graph G627 the equivalent index of the effective index of thecore 110C3 located around the reference core 110A1.

Based on the above discussion, let us consider in such a manner that theradial departure r from the reference core due to bending, which wasconsidered to be the radial departure r from the center core to eachcore with the center core being defined as a reference core, is replacedwith a distance between different types of cores. In this case, when thecore distance between nonidentical cores in the cross section of themulti-core fiber is D and when a radius of curvature permitted in termsof crosstalk is R, the relative refractive-index difference Δ_(eff)between the actual effective index in one type of core (actual effectiveindex without conversion to equivalent index) and the actual effectiveindex in another type of core needs to satisfy at least the condition offormula (13) below, for every pair of nonidentical cores.

$\begin{matrix}{{\Delta_{eff} \geq {\Delta_{eq} + \alpha}} = {\frac{{2\frac{D}{R}} + ( \frac{D}{R} )^{2}}{2( {1 + \frac{D}{R}} )^{2}} + \alpha}} & (13)\end{matrix}$

It is noted herein that a in the above formula (13) is the relativerefractive-index difference between effective indices of nonidenticalcores (with different refractive indices), in the case wheresufficiently low crosstalk can be realized by the multi-core fiberdesigned without consideration to bending. The above formula (13)defines the relative refractive-index difference of the high effectiveindex from the low effective index so as to satisfy Δ_(eff)>0, whileselecting the reference core so as to satisfy Δ_(eq)>0.

According to above Document 1, the sufficient difference of core Δ withthe core distance D=30 μm between adjacent cores is 0.005%, and thus asufficient amount of the above parameter α is also 0.005%; therefore,the relative refractive-index difference Δ_(eff) needs only to satisfyformula (14) below in percentage expression. This allows crosstalkbetween cores to be controlled at a low level even with a bend of notless than the radius of curvature R.

$\begin{matrix}{\Delta_{eff} \geq {{\frac{{2\frac{D}{R}} + ( \frac{D}{R} )^{2}}{2( {1 + \frac{D}{R}} )^{2}} \cdot 100} + 0.005}} & (14)\end{matrix}$

A multi-core fiber composed of a plurality of cores can be one in whichthere are a plurality of cores in each of different types. In themulti-core fiber of this kind, the cores of the same type are arrangedin a state in which a sufficient core distance D is ensured so as todecrease crosstalk. Therefore, when the minimum core distance betweenidentical cores is D_(min) and when the core distance D betweennonidentical cores exceeds D_(min), there is no need for givingconsideration to the relative refractive-index differences betweeneffective indices of these nonidentical cores (because crosstalk issufficiently low even between identical cores with an equal effectiveindex). However, at least formula (15) below needs to be satisfied forall combinations between nonidentical cores with the core distance Dbeing less than D_(min). The reason for it is that the conversion ofeffective index to equivalent index is not equal for each combination ofnonidentical cores with the core distance D being shorter than D_(min).When this condition is satisfied, crosstalk between cores can becontrolled at a low level even with a bend of not less than the radiusof curvature R.

$\begin{matrix}{\Delta_{eff} > {\frac{{2\frac{D}{R}} + ( \frac{D}{R} )^{2}}{2( {1 + \frac{D}{R}} )^{2}} \cdot 100}} & (15)\end{matrix}$

However, if the multi-core fiber satisfying formula (14) or formula (15)as described above permits the parameter R=30 mm, the relativerefractive-index difference Δ_(eff) needs to be not less than 0.105%(Δ_(eff)≧0.0105%) where the core distance D=30 μm. It is not easy tomeet this requirement. Specifically, it requires some means to give alarge difference in core Δ or core diameter between cores in themulti-core fiber 100A, or to provide a difference from the refractiveindex of the surrounding cladding between different types of cores.

The reason why the inter-core crosstalk becomes large is that thedifference between equivalent indices of effective indices of coresbecomes very small between cores. However, if areas where the differencebecomes small below a certain level are very small along thelongitudinal direction of the multi-core fiber 100A, the inter-corecrosstalk is considered to become small as well. Then the first andsecond embodiments will be described below in order.

First Embodiment

First, in the first embodiment, the relation of formula (16) belowholds, where out of the plurality of cores in the multi-core fiber 100A,n_(eff-m) is the effective index of core m, n_(eqeff-nm) the equivalentindex of the effective index of core n on the basis of the core m,D_(nm) the core distance (intercentral distance) between core n and corem, and φ_(nm) (rad) an angle between a straight line mn and a straightline agreeing with the bending radius direction of the multi-core fiber100A. The straight line mn means a line connecting the center of thecore m and the center of the core n on a cross section of the multi-corefiber 100A perpendicular to the predetermined axis AX.

$\begin{matrix}{n_{{eqeff} - {nm}} = {n_{{eff} - n}\{ {1 + \frac{D_{nm}\cos \; \theta_{nm}}{R}} \}}} & (16)\end{matrix}$

When formula (16) above is defined with propagation constants in placeof the effective indices, we obtain formula (17) below becauseβ=(2π/λ)n_(eff) (where λ is the wavelength and n_(eff) the effectiveindex).

$\begin{matrix}{\beta_{{eq} - {nm}} = {\beta_{n}\{ {1 + \frac{D_{nm}\cos \; \theta_{nm}}{R}} \}}} & (17)\end{matrix}$

In the above formula, β_(n) is the propagation constant of the core n,and β_(eq-nm) the propagation constant of the core n taking account ofthe equivalent index on the basis of the core m.

At this time, a difference Δβ_(nm) between β_(eq-nm) and β_(eq-nn)(which is not the relative refractive-index difference) is given byformula (18) below.

$\begin{matrix}\begin{matrix}{{\Delta \; \beta_{nm}} = {\beta_{{eq} - {nm}} - \beta_{{eq} - {mm}}}} \\{= {{\beta_{n}\{ {1 + \frac{D_{nm}\cos \; \theta_{nm}}{R}} \}} - \beta_{m}}} \\{= {{\beta_{n}\frac{D_{nm}\cos \; \theta_{nm}}{R}} + ( {\beta_{n} - \beta_{m}} )}}\end{matrix} & (18)\end{matrix}$

It is considered that the inter-core crosstalk decreases as a rate ofΔβ_(nm) being close to 0 becomes smaller along the longitudinaldirection of the multi-core fiber. When the parameter R=30 mm ispermitted, it is not easy to prevent the difference Δβ_(nm) from alwaysbecoming 0, while the core distance D_(nm) between core n and core m isequal to 30 μm. Namely, it is because it becomes necessary to make thedifference between the propagation constant β_(n) and the propagationconstant β_(m) such that the relative refractive-index differenceΔ_(eff) between effective indices exceeds 0.1%, as shown in FIG. 3B.

It is therefore considered that a desired situation is such that thereare zeros of Δβ_(nm) along the longitudinal direction of the multi-corefiber, but a slope of Δβ_(nm) at each zero is steep and the frequency ofappearance of zeros is low. Particularly, it is important that the slopeof Δβ_(nm) at each zero be steep.

FIG. 7 is a graph showing variation in inter-core crosstalk (which isreferred to simply as “crosstalk” in FIG. 7) along the longitudinaldirection of a multi-core fiber with two cores (which will be referredto hereinafter as a 2-core fiber), which is specifically variationappearing along the longitudinal direction of the 2-core fiber and withincidence of light with the optical intensity I₁=1 into one of the twocores, in the optical intensity I₂ of the other core. When theinter-core crosstalk is defined as (intensity of a certain non-incidentcore)/(total of intensities of all cores), the graph of FIG. 7 can besaid to be a graph of variation in crosstalk along the longitudinaldirection of the 2-core fiber. In this 2-core fiber, a fixed bend isgiven throughout the entire length. Furthermore, the fiber is given atwist along the longitudinal direction of the 2-core fiber(unidirectional rotation about the axis of the 2-core fiber). This twistprovides the 2-core fiber with one rotation per 10 m. Namely, when zstands for the longitudinal position of the 2-core fiber, there are twozeros of Δβ_(nm)(z) per 10 m. In FIG. 7, steep changes of crosstalkexisting at equal intervals and at a rate of two per 10 m are zeros ofΔβ_(nm)(z).

The above-described simulation was to calculate the variation ofinter-core crosstalk, and then expressions to express the behavior ofcrosstalk more simply will be established below.

The inverse of the slope given by formula (19a) below, at an arbitraryzero z of Δβ_(nm)(z) can be used as an index indicating how longΔβ_(nm)(z) is located near 0 in passing the zero z. Then a crosstalkamount χ between cores at the arbitrary zero is expressed with an indexof formula (19b) below and the crosstalk amount χ between cores isconsidered to decrease as the value of this parameter 1 becomes smaller.

$\begin{matrix}{\frac{}{z}\Delta \; {\beta_{nm}(z)}} & ( {19a} ) \\{l = {\frac{1}{{\frac{}{z}\Delta \; {\beta_{nm}(z)}}_{{\Delta \; {\beta_{nm}{(z)}}} = 0}}}} & ( {19b} )\end{matrix}$

Furthermore, it is considered that significant inter-core crosstalkoccurs near extremes of zeros z only. When aforementioned formula (10)and formula (11) are considered herein, F=1 and L=(π/2)·(1/κ) from therelation of formula (20a) below. When coupling between two cores isconsidered, in the case where F=1 and L=(π/2)·(1/κ) and where light withthe intensity I₁=1 is incident into one core 1, formula (20b) belowrepresents the intensity I₂ at the longitudinal position z of the 2-corefiber, in the other core 2.

$\begin{matrix}{\psi = {{\Delta \; {\beta_{21}/2}} = 0}} & ( {20a} ) \\{I_{2} = {\sin^{2}( {\frac{\kappa}{2\pi}z} )}} & ( {20b} )\end{matrix}$

Further supposing I₁>>I₂, z in the above formula (20b) can be assumed tobe near 0, in the vicinity of each zero of Δβ_(nm)(z). Then theintensity I₂ can be approximated by formula (21) below.

$\begin{matrix}{I_{2} \approx {( \frac{\kappa}{2\pi} )^{2}z^{2}}} & (21)\end{matrix}$

Furthermore, with consideration including the fact that the respectivevalues of F and L also gradually change with deviation from a zero ofΔβ_(nm)(z), the crosstalk amount χ between cores near an arbitrary zeroof Δβ_(nm)(z) is considered to be expressed by formula (22) below,eventually in the case of I₁>>I₂.

$\begin{matrix}{\chi = {( \frac{\kappa_{nm}}{2\pi} )^{2}\alpha {\frac{1}{{\frac{}{z}\Delta \; {\beta_{nm}(z)}}_{{\Delta \; {\beta_{nm}{(z)}}} = 0}}}}} & (22)\end{matrix}$

In the above formula, α is a coefficient to relate above formula (19b)to above formula (21).

The inter-core crosstalk amount χ will be determined below for somecases.

θ_(nm) is a function of z among the parameters in formula (18) above andlet us consider a case where the relation of formula (23) below holds(where γ_(c)≠0).

θ_(nm)(z)=γ_(c) z  (23)

In this case, where the longitudinal position z of the 2-core fiber isgiven by formula (24a) below, Δβ_(nm)(z)=0; the relation represented byformula (24b) below holds at any point and the inter-core crosstalkamount χ at any point is given by formula (24c) below.

$\begin{matrix}{z = {{\pm \frac{1}{\gamma_{c}}}\{ {{a\; {\cos ( {\frac{R}{D_{nm}}\frac{\beta_{m} - \beta_{n}}{\beta_{n}}} )}} + {2\pi \; k}} \}}} & ( {24a} )\end{matrix}$

(where k is an integer and a range of a cos (x) is [0,π].)

$\begin{matrix}{{{\frac{}{z}\Delta \; {\beta_{nm}(z)}}} = {\beta_{n}{\gamma_{c}}\sqrt{( \frac{D_{nm}}{R} )^{2} - ( \frac{\beta_{m} - \beta_{n}}{\beta_{n}} )^{2}}}} & ( {24b} ) \\{\chi = {{\alpha ( \frac{\kappa_{nm}}{2\pi} )}^{2}\frac{1}{\beta_{n}}\frac{1}{\gamma_{c}}\frac{1}{\sqrt{( \frac{D_{nm}}{R} )^{2} - ( \frac{\beta_{m} - \beta_{n}}{\beta_{n}} )^{2}}}}} & ( {24c} )\end{matrix}$

In the case of a relation represented by formula (25a) below (whereγ_(a)≧π and γ_(f)>0), a relation represented by formula (25c) belowholds at the longitudinal position z of the 2-core fiber whereΔβ_(nm)(z)=0 (formula (25b) below), and the inter-core crosstalk amountχ is given by formula (25d) below.

$\begin{matrix}{{\theta_{nm}(z)} = {\gamma_{a}{\cos ( {\gamma_{f}z} )}}} & ( {25a} ) \\{z = {\frac{1}{\gamma_{f}}\{ {{{\pm a}\; {\cos ( {\frac{1}{\gamma_{a}}\{ {{{\pm a}\; {\cos ( {\frac{R}{D_{nm}}\frac{\beta_{m} - \beta_{n}}{\beta_{n}}} )}} + {2\pi \; k_{1}}} \}} )}} + {2\pi \; k_{3}}} \}}} & ( {25b} )\end{matrix}$

(where double signs are arbitrary and each of k₁ and k₂ is an integer ina range satisfying the domain of the arccosine function in the formula.)

$\begin{matrix}{{{\frac{}{z}\Delta \; {\beta_{nm}(z)}}} = {\beta_{n}\gamma_{f}\sqrt{( \frac{D_{nm}}{R} )^{2} - ( \frac{\beta_{m} - \beta_{n}}{\beta_{n}} )^{2}}\sqrt{\gamma_{a}^{2} - \{ {{{\pm a}\; {\cos ( {\frac{R}{D_{nm}}\frac{\beta_{m} - \beta_{n}}{\beta_{n}}} )}} + {2\pi \; k_{1}}} \}^{2}}}} & ( {25c} ) \\{\chi = {{\alpha ( \frac{\kappa_{nm}}{2\pi} )}^{2}\frac{1}{\beta_{n}}\frac{1}{\gamma_{f}}\frac{1}{\sqrt{( \frac{D_{nm}}{R} )^{2} - ( \frac{\beta_{m} - \beta_{n}}{\beta_{n}} )^{2}}}\frac{1}{\sqrt{\gamma_{a}^{2} - \{ {{{\pm a}\; {\cos ( {\frac{R}{D_{nm}}\frac{\beta_{m} - \beta_{n}}{\beta_{n}}} )}} + {2\pi \; k_{1}}} \}^{2}}}}} & ( {25d} )\end{matrix}$

(where each of k₁ and k₂ is an integer in a range satisfying the domainof the arccosine function in the formulae.)

From the above discussion, in order to decrease the inter-core crosstalkamount χ in the 2-core fiber, it is necessary to increase the coredistance D_(nm) between the two cores n and m, to decrease the parameterR (the radius of curvature for the 2-core fiber), or to decrease thedifference between the propagation constant β_(n) of the core n and thepropagation constant β_(m) of the core m (i.e., to decrease thedifference between n_(eff-n) and n_(eff-m)). Particularly, an increaseof the core distance D_(nm) between core n and core m leads naturally toa decrease of the coupling coefficient κ between cores, so as to achievea significant effect of reduction in crosstalk between cores.Furthermore, the inter-core crosstalk amount χ can also be decreased byincreasing the parameters γ_(c) and γ_(f).

As also seen from the above description, it is desirable to keepn_(eff-n)=n_(eff-m), in terms of the inter-core crosstalk amount aswell, and it is also readily feasible in terms of manufacture to achievethe multi-core fiber 100A because it can be manufactured in the samecore structure. In the description hereinafter, therefore, the case ofn_(eff-n)=n_(eff-m) will be discussed.

When n_(eff-n)=n_(eff-m), the above formula (24c) can be expressed byformula (26a) below and the above formula (24d) by formula (26b) below.

$\begin{matrix}{\chi = {{\alpha ( \frac{\kappa_{nm}}{2\pi} )}^{2}\frac{1}{\beta_{n}}\frac{1}{\gamma_{c}}\frac{R}{D_{nm}}}} & ( {26a} ) \\{{\chi = {{\alpha ( \frac{\kappa_{nm}}{2\pi} )}^{2}\frac{1}{\beta_{n}}\frac{1}{\gamma_{f}}\frac{R}{D_{nm}}\frac{1}{\sqrt{\gamma_{a}^{2} - ( {\pi \; k} )^{2}}}}}( {{{{where}\mspace{14mu} k\mspace{14mu} {is}\mspace{14mu} {an}\mspace{14mu} {integer}\mspace{14mu} {satisfying}} - \frac{\gamma_{a}}{\pi}} \leq k \leq {\frac{\gamma_{a}}{\pi}.}} )} & ( {26b} )\end{matrix}$

Now, let us consider another method for the inter-core crosstalk amountχ in the 2-core fiber. For simplicity, let us consider the case of theabove formula (26a).

By letting A be the complex electric field amplitude by slowly varyingenvelope approximation, a coupled-mode equation from the core m to thecore n is expressed by formula (27) below.

$\begin{matrix}{\frac{\partial A_{n}}{\partial z} = {{- j}\; \kappa_{nm}{\exp ( {{- j}\{ {{\varphi_{m}(z)} - {\varphi_{n}(z)}} \}} )}A_{m}}} & (27)\end{matrix}$

Furthermore, when a twist of optical fiber is represented by γ_(c)(rad/m), the coupled-mode equation is represented by formula (28) below.

$\begin{matrix}\{ {{\begin{matrix}{{\varphi_{m}(z)} = {\beta_{m}z}} \\{{\varphi_{n}(z)} = {\int_{0}^{z}{\beta_{n}\{ {1 + {\frac{D_{nm}}{R}\cos \; {\theta_{n}( z^{\prime} )}}} \} \ {z^{\prime}}}}}\end{matrix}{\theta_{n}(z)}} = {\gamma_{c}z}}  & (28)\end{matrix}$

It is assumed in formula (28) above that β_(m), β_(n), D_(nm), and R arein a relation in which the equivalent effective indices of the core nand the core m can become equal depending upon the position of z.Usually, the complex electric field amplitude A_(m) of the core m varieslongitudinally because of coupling from the core n to the core m. Forthis reason, it is difficult to obtain an analytical solution of thecomplex electric field amplitude A_(n) of the core n, but when weconsider the case where the crosstalk is sufficiently small, A_(m) canbe approximated to 1. At this time, an integration represented byformula (29) below can be established.

A _(n)(z)=−jκ _(nm)∫₀ ^(z)exp(−j{φ _(m)(z′)−φ_(n)(z′)})dz′  (29)

In consideration of the above formula (28) and the collateral conditionsfor the respective variables in this formula (28), there is always onepoint where the equivalent effective indices of the core n and the corem become equal during a change of z from 0 to π/γ_(c). Then thecrosstalk amount χ can be represented by formula (30) below.

$\begin{matrix}{\chi = {{A_{n}( \frac{\pi}{\gamma_{c}} )}}^{2}} & (30)\end{matrix}$

Formula (31) below provides the result obtained by solving the aboveformula (30) with respect to A_(n)(π/γ_(c)).

$\begin{matrix}\begin{matrix}{{A_{n}( \frac{\pi}{\gamma_{c}} )} = {j\; \kappa_{nm}{\int_{0}^{\pi/\gamma_{c}}{{\exp ( {{- j}\{ {{\varphi_{m}( z^{\prime} )} - {\varphi_{n}( z^{\prime} )}} \}} )}\ {z^{\prime}}}}}} \\{= {{- j}\; \kappa_{nm}{\int_{0}^{\pi/\gamma_{c}}{{\exp ( {{- j}\begin{Bmatrix}{{\beta_{m}z^{\prime}} -} \\\begin{pmatrix}{{\beta_{n}z^{\prime}} +} \\{\beta_{n}\frac{D_{nm}}{\gamma_{c}R}{\sin ( {\gamma_{c}z^{\prime}} )}}\end{pmatrix}\end{Bmatrix}} )}{z^{\prime}}}}}} \\{= {{- j}\; \kappa_{nm}{\int_{0}^{\pi/\gamma_{c}}{\exp \{ {{- {j( {\beta_{m} - \beta_{n}} )}}z^{\prime}} \}}}}} \\{{\exp \; \{ {j\frac{\beta_{n}D_{nm}}{\gamma_{c}R}{\sin ( {\gamma_{c}z^{\prime}} )}} \} {z^{\prime}}}} \\{= {{- j}\; \kappa_{nm}{\int_{0}^{\pi/\gamma_{c}}{\exp \{ {{- {j( {\beta_{m} - \beta_{n}} )}}z^{\prime}} \}}}}} \\{{\sum\limits_{v}{J_{v}\; ( \frac{\beta_{n}D_{nm}}{\gamma_{c}R} ){\exp ( {j\; v\; \gamma_{c}z^{\prime}} )}{z^{\prime}}}}} \\{= {{- j}\; \kappa_{nm}{\sum\limits_{v}{\int_{0}^{\pi/\gamma_{c}}{{J_{v}( \frac{\beta_{n}D_{nm}}{\gamma_{c}R} )}\exp}}}}} \\{{\{ {{- {j( {\beta_{m} - \beta_{n} - {v\; \gamma_{c}}} )}}z^{\prime}} \} \ {z^{\prime}}}} \\{= {{- j}\; \kappa_{nm}\begin{Bmatrix} {\frac{\pi}{\gamma_{c}}{J_{v}( \frac{\beta_{n}D_{nm}}{\gamma_{c}R} )}} \middle| {}_{{\beta_{m} - \beta_{n} - {v\; \gamma_{c}}} = 0} +  \\\begin{matrix}\sum\limits_{{\beta_{m} - \beta_{n} - {v\; \gamma_{c}}} \neq 0} \\\lbrack {{J_{v}( \frac{\beta_{n}D_{nm}}{\gamma_{c}R} )}\frac{\exp \{ {{- {j( {\beta_{m} - \beta_{n} - {v\; \gamma_{c}}} )}}z^{\prime}} \}}{- {j( {\beta_{m} - \beta_{n} - {v\; \gamma_{c}}} )}}} \rbrack_{0}^{\frac{\pi}{\gamma_{c}}}\end{matrix}\end{Bmatrix}}}\end{matrix} & (31)\end{matrix}$

When the relation of β_(m)=β_(n) is met, the above formula (31) can berewritten into formula (32) below.

$\begin{matrix}{{A_{n}( \frac{\pi}{\gamma_{c}} )} = {{- j}\frac{\kappa_{nm}}{\gamma_{c}}\{ {{{J_{0}( \frac{\beta_{n}D_{nm}}{\gamma_{c}R} )}\pi} + {j{\sum\limits_{v \neq 0}{\frac{( {- 1} )^{v} - 1}{v}{J_{v}( \frac{\beta_{n}D_{nm}}{\gamma_{c}R} )}}}}} \}}} & (32)\end{matrix}$

Furthermore, by using the relation of formula (33) below described inDocument 3 (Shigeichi Moriguchi et al., “Iwanami Suugaku Kousiki(Mathematical Formulae) III,” p. 154, Iwanami Shoten (1987)), the aboveformula (32) can be modified as described in formula (34) below.

$\begin{matrix}{{J_{v}(x)} \approx {\sqrt{\frac{2}{\pi \; x}}{{\cos ( {x - {\frac{{2v} + 1}{4}\pi}} )}\mspace{14mu}\lbrack {x\operatorname{>>}1} \rbrack}}} & (33) \\\begin{matrix}{{A_{n}( \frac{\pi}{\gamma_{c}} )} = {{- j}\frac{\kappa_{nm}}{\gamma_{c}}\begin{Bmatrix}{{\sqrt{2\pi \frac{\gamma_{c}R}{\beta_{n}D_{nm}}}{\cos ( {\frac{\beta_{n}D_{nm}}{\gamma_{c}R} - \frac{\pi}{4}} )}} +} \\\begin{matrix}{j\sqrt{\frac{2}{\pi}\frac{\gamma_{c}R}{\beta_{n}D_{nm}}}{\sum\limits_{v \neq 0}\frac{( {- 1} )^{v} - 1}{v}}} \\{\cos ( {\frac{\beta_{n}D_{nm}}{\gamma_{c}R} - {\frac{{2v} + 1}{4}\pi}} )}\end{matrix}\end{Bmatrix}}} \\{= {{- j}\frac{\kappa_{nm}}{\gamma_{c}}\begin{Bmatrix}{{\sqrt{2\pi \frac{\gamma_{c}R}{\beta_{n}D_{nm}}}{\cos ( {\frac{\beta_{n}D_{nm}}{\gamma_{c}R} - \frac{\pi}{4}} )}} +} \\{\frac{j}{\pi}{\sum\limits_{v \neq 0}{\frac{( {- 1} )^{v} - 1}{v}{\cos ( {\frac{\beta_{n}D_{nm}}{\gamma_{c}R} - {\frac{{2v} + 1}{4}\pi}} )}}}}\end{Bmatrix}}}\end{matrix} & (34)\end{matrix}$

Here let us consider the imaginary term (summation term) in theright-side parentheses in the above formula (34). First, the imaginaryterm in the above formula (34) can be modified by making use of therelation of formula (35) below.

$\begin{matrix}\begin{matrix}{{\sum\limits_{v \neq 0}{\frac{( {- 1} )^{v} - 1}{v}{\cos ( {x - {\frac{{2v} + 1}{4}\pi}} )}}} = {\sum\limits_{v \neq 0}\frac{( {- 1} )^{v} - 1}{v}}} \\{\begin{Bmatrix}{{{\cos ( {x - \frac{\pi}{4}} )}{\cos ( {\frac{v}{2}\pi} )}} +} \\{\sin ( {x - \frac{\pi}{4}} ){\sin ( {\frac{v}{2}\pi} )}}\end{Bmatrix}} \\{= {{\cos ( {x - \frac{\pi}{4}} )}{\sum\limits_{v \neq 0}\frac{( {- 1} )^{v} - 1}{v}}}} \\{{{\cos ( {\frac{v}{2}\pi} )} + {\sin ( {x - \frac{\pi}{4}} )}}} \\{{\sum\limits_{v \neq 0}{\frac{( {- 1} )^{v} - 1}{v}{\sin ( {\frac{v}{2}\pi} )}}}}\end{matrix} & (35)\end{matrix}$

At this time, since the first term of the right side is an odd functionwith respect to v and is thus 0. Furthermore, since the second term ofthe right side is an even function with respect to v, it can be arrangedby making use of formula (36) below described in Document 4 (ShigeichiMoriguchi et al., “Iwanami Suugaku Kousiki (Mathematical Formulae) II,”p. 72, Iwanami Shoten (1987)) and can be expressed by formula (37)below.

$\begin{matrix}{{\sum\limits_{n = 1}^{\infty}\frac{\sin \{ {( {{2n} - 1} )x} \}}{{2n} - 1}} = \{ \begin{matrix}{\pi/4} & \lbrack {0 < x < \pi} \rbrack \\0 & \lbrack {x = \pi} \rbrack \\{{- \pi}/4} & \lbrack {\pi < x < {2n}} \rbrack\end{matrix} } & (36) \\\begin{matrix}{{{\sin ( {x - \frac{\pi}{4}} )}{\sum\limits_{v \neq 0}{\frac{( {- 1} )^{v} - 1}{v}{\sin ( {\frac{v}{2}\pi} )}}}} = {2{\sin ( {x - \frac{\pi}{4}} )}{\sum\limits_{v = 1}^{\infty}\frac{( {- 1} )^{v} - 1}{v}}}} \\{{\sin \{ {\frac{v}{2}\pi} \}}} \\{= {2{\sin ( {x - \frac{\pi}{4}} )}{\sum\limits_{v^{\prime} = 1}^{\infty}\frac{- 2}{{2v^{\prime}} - 1}}}} \\{{\sin \{ {( {{2v^{\prime}} - 1} )\frac{\pi}{2}} \}}} \\{= {{{- 2} \cdot 2}{\sin ( {x - \frac{\pi}{4}} )}}} \\{{\sum\limits_{v^{\prime} = 1}^{\infty}\frac{\sin \{ {( {{2v^{\prime}} - 1} ){\pi/2}} \}}{{2v^{\prime}} - 1}}} \\{= {{- \pi}\; {\sin ( {x - \frac{\pi}{4}} )}}}\end{matrix} & (37)\end{matrix}$

By using the formula (35) and formula (37) obtained as described above,the above formula (34) can be arranged like formula (38) below.

$\begin{matrix}\begin{matrix}{{A_{n}( \frac{\pi}{\gamma_{c}} )} = {{- j}\frac{\kappa_{nm}}{\gamma_{c}}\sqrt{2\pi \frac{\gamma_{c}R}{\beta_{n}D_{nm}}}\begin{Bmatrix}{{\cos ( {\frac{\beta_{n}D_{nm}}{\gamma_{c}R} - \frac{\pi}{4}} )} -} \\{j\; {\sin ( {\frac{\beta_{n}D_{nm}}{\gamma_{c}R} - \frac{\pi}{4}} )}}\end{Bmatrix}}} \\{= {\frac{\kappa_{nm}}{\gamma_{c}}\sqrt{2\pi \frac{\gamma_{c}R}{\beta_{n}D_{nm}}}{\exp \lbrack {- {j( {\frac{\beta_{n}D_{nm}}{\gamma_{c}R} - \frac{\pi}{4}} )}} \rbrack}}}\end{matrix} & (38)\end{matrix}$

Therefore, based on the above formula (30), the crosstalk amount χ canbe determined as in formula (39) below.

$\begin{matrix}{\chi = {\frac{\kappa_{nm}^{2}}{\beta_{n}}\frac{R}{D_{nm}}\frac{2\pi}{\gamma_{c}}}} & (39)\end{matrix}$

Since the above formula (39) is equal to the above formula (26a), theresult of formula (40) below can be derived.

α=(2π)′  (40)

FIG. 8 shows a relation of the crosstalk amount χ between the analyticalsolution of the above formula (39) and values obtained by simulationbased on the coupled-mode equation.

The results shown are based on calculations for all combinations of thewavelength of 1.55 μm, core Δ of 0.3% and 0.4%, R of 60 mm, 120 mm, 180mm, 240 mm, and 300 mm, and D_(nm) of 35 μm and 40 μm. Good agreement isachieved between the analytical solution and the simulation results, soas to mutually confirm correctness of the analytical solution andcorrectness of the simulation.

Incidentally, since the crosstalk amount χ is the crosstalk variationamount at zeros of the equivalent propagation constant differencebetween cores, it is seen in terms of change in complex electric fieldamplitude that the relation of formula (41) below holds under assumptionof low crosstalk. A_(n)(n_(zero)) in this formula (41) is A_(n) afterpassage of n_(zero) zeros of the equivalent propagation constantdifference. φ_(random) is arg(jA_(n)/A_(n)) at each zero and takesrandom values at respective zeros due to variation in γ_(c) and R infact, and therefore, it is denoted as in the below formula.

A _(n)(n _(zero)+1)=A _(n)(n _(zero))+√{square root over (χ)}exp(jφ_(random))  (41)

Since two values represented by formula (42a) below follow theprobability distribution of σ²=χ/2, if n_(zero) is sufficiently large,two values in formula (42b) below are distributed as probabilitydistributions of normal distributions being stochastically independentof each other and having an identical variance of σ²=(χ/2)×n_(zero), bythe central limit theorem. n_(zero) is intrinsically an integer, but ifthe above formula (25c) holds, it can be replaced as in formula (42c)below.

$\begin{matrix}{{\Re \{ {\sqrt{\chi}{\exp ( {j\; \varphi_{random}} )}} \}},{\{ {\sqrt{\chi}{\exp ( {j\; \varphi_{random}} )}} \}}} & ( {42a} ) \\{{\Re \{ {A_{n}( n_{zero} )} \}},{\{ {A_{n}( n_{zero} )} \}}} & ( {42b} ) \\{n_{zero} = {\frac{\gamma_{c}}{\pi}L_{F}}} & ( {42c} )\end{matrix}$

In this case, σ² satisfies formula (43) below. L_(F) is a fiber length.

$\begin{matrix}{\sigma^{2} = {\frac{\kappa_{nm}^{2}}{\beta_{n}}\frac{R}{D_{nm}}L_{F}}} & (43)\end{matrix}$

Since two polarization modes have to be considered in fact, therespective values of formula (42b) of the two polarization modes satisfyformula (44) below.

$\begin{matrix}{\sigma^{2} = {\frac{1}{2}\frac{\kappa_{nm}^{2}}{\beta_{n}}\frac{R}{D_{nm}}L_{F}}} & (44)\end{matrix}$

Values indicated in formula (45a) below are distributed according toformula (45b) below which is a chi-square distribution with four degreesof freedom, a cumulative distribution function thereof is further givenby formula (45c) below, and an average XT_(μ) of the distribution isgiven by formula (45d) below.

$\begin{matrix}\frac{{{A_{n}( n_{zero} )}}^{2}}{\sigma^{2}} & ( {45a} ) \\{{f(x)} = {\frac{1}{4}x\; {\exp ( {- \frac{x}{2}} )}}} & ( {45b} ) \\{{F(x)} = {1 - {( {1 + \frac{x}{2}} ){\exp ( {- \frac{x}{2}} )}}}} & ( {45c} ) \\{{XT}_{\mu} = {{4\sigma^{2}} = {2\frac{\kappa^{2}}{\beta}\frac{R}{D_{nm}}L_{F}}}} & ( {45d} )\end{matrix}$

In order to keep the average XT_(μ) of the crosstalk distribution notmore than a tolerance XT_(S), relations of formulae (46b) to (46d) beloware obtained from a relation of formula (46a) below.

$\begin{matrix}{{XT}_{\mu} = {{2\frac{\kappa^{2}}{\beta}\frac{R}{D_{nm}}L_{F}} \leq {XT}_{S}}} & ( {46a} ) \\{{\kappa \leq \sqrt{\frac{1}{2}\beta \frac{D_{nm}}{R}\frac{{XT}_{S}}{L_{F}}}} = \kappa_{th}} & ( {46b} ) \\{{D_{nm} \geq {2\frac{\kappa^{2}}{\beta}R\frac{L_{F}}{{XT}_{S}}}} = D_{{nm} - {th}}} & ( {46c} ) \\{{R \leq {\frac{1}{2}\frac{\beta}{\kappa^{2}}D_{nm}\frac{{XT}_{S}}{L_{F}}}} = R_{th}} & ( {46d} )\end{matrix}$

By giving XT_(S) and L_(F), relational expressions to be satisfied bythe respective parameters become evident. In the case of a multi-coreoptical fiber with a plurality of cores of the same structure, if thefiber is designed with the coupling coefficient of not more thanκ_(nm-th) and the inter-core distance of not less than D_(nm-th) and ifthe fiber is bent with a radius of not more than R_(th), the crosstalkcan be controlled at a level of not more than XT_(S).

Now, let us consider an optical fiber with seven cores #1 to #7 as shownin FIG. 9 (which will be referred to hereinafter as “7-core opticalfiber”). Since the coupling coefficient between cores exponentiallydecreases against core distance, we can think that the coresnecessitating consideration to crosstalk are only adjacent cores. Inthis case, core 1 with the largest number of adjacent cores is affectedby crosstalk from six cores located around it. At this time, when thecore pitch is denoted by Λ, the above formulae (46a)-(46d) can berewritten into formulae (47a)-(47d) below, respectively. In cases wherethe number of cores is not less than 7, as long as the cores arearranged in a hexagonal lattice pattern, the formulae to be consideredare the formulae (47a)-(47d) below.

$\begin{matrix}{{XT}_{\mu} = {{{6 \cdot 2}\frac{\kappa^{2}}{\beta}\frac{R}{\Lambda}L_{F}} \leq {XT}_{S}}} & ( {47a} ) \\{{\kappa \leq \sqrt{\frac{1}{12}\beta \frac{\Lambda}{R}\frac{{XT}_{S}}{L_{F}}}} = \kappa_{th}} & ( {47b} ) \\{{\Lambda \geq {12\frac{\kappa^{2}}{\beta}R\frac{L_{F}}{{XT}_{S}}}} = \Lambda_{th}} & ( {47c} ) \\{{R \leq {\frac{1}{12}\frac{\beta}{\kappa^{2}}\Lambda \frac{{XT}_{S}}{L_{F}}}} = R_{th}} & ( {47d} )\end{matrix}$

Since XT_(μ) generally requires consideration to the above formula(24c), the above formula (40), the relation of σ²=(χ/2)×n_(zero), theabove formula (42c), and the two polarization modes, the averageXT_(μ,n) of the crosstalk distribution to core n can be expressed byformula (48) below.

$\begin{matrix}{{XT}_{\mu,n} = {\sum\limits_{m \neq n}{2\frac{\kappa_{nm}^{2}}{\beta_{m}}\frac{R}{D_{nm}}L_{F}\frac{1}{\sqrt{1 - ( {\frac{R}{D_{nm}}\frac{\beta_{n} - \beta_{m}}{\beta_{m}}} )^{2}}}}}} & (48)\end{matrix}$

As the radius of curvature R of the optical fiber becomes smaller, thecore pitch Λ can also be made smaller, thereby increasing the coredensity per unit area of the fiber cross section. Since optical fibersare usually used in a cable form, the optical fibers are housed in thecable, for example, in such a manner that they are arranged at a fixeddistance from the center of the cable on the cable cross section andthat directions of the optical fibers from the cable center vary withchange in the longitudinal position of the cable. This arrangementallows the optical fibers to be maintained in a helix shape and with avirtually constant radius of curvature even if the cable is in astraight state.

When the optical fibers are housed in the helix shape in the cable asdescribed above, the fiber length increases with respect to the cablelength. In this case, where the radius of the helix is r_(h) and thepitch thereof is L_(P) as shown in FIG. 10, the radius of curvature R ofthe helix is represented by formula (49) below. FIG. 10 is a view forexplaining the radius r_(h) and the pitch L_(P) of the helix.

$\begin{matrix}{R = \frac{r_{h}^{2} + ( \frac{L_{P}}{2\pi} )^{2}}{r_{h}}} & (49)\end{matrix}$

An increase rate L_(D) of fiber length to cable length is represented byformula (50) below.

$\begin{matrix}{L_{D} = {\sqrt{( {r_{h}\frac{2\pi}{L_{P}}} )^{2} + 1} - 1}} & (50)\end{matrix}$

Therefore, a relation between L_(D) and R can be expressed by formula(51) below.

$\begin{matrix}{L_{D} = {\sqrt{\frac{R}{R - r_{h}}} - 1}} & (51)\end{matrix}$

Hence, an increase α_(D) of loss per span due to L_(D) can berepresented by formula (52) and formula (53) below, where L_(span) (km)is a span length and α_(km) (dB/km) is an attenuation coefficient perkm.

$\begin{matrix}{\alpha_{D} = {{\{ {\sqrt{( {r_{h}\frac{2\pi}{L_{P}}} )^{2} + 1} - 1} \} \cdot \alpha_{km}}L_{span}}} & (52) \\{\alpha_{D} = {{\lbrack {\sqrt{\frac{R}{R - r_{h}}} - 1} \rbrack \cdot \alpha_{km}}L_{span}}} & (53)\end{matrix}$

For the ordinary cables existing presently, r_(h) is not more than 12 mmand L_(P) not less than 300 mm. Under such circumstances, if L_(span) is80 km and α_(km) is 0.185 dB/km, α_(D) is at most 0.305 dB/span.

With consideration to degradation of OSNR during transmission, it isdesirable to keep α_(D) not more than a tolerance α_(S). Therefore, theconditions to be satisfied by L_(P) and R are determined to be formula(54) and formula (55) below from the above formulae (52) and (53), andthe relation of α_(D)≦α_(S). It is noted that the largest value of r_(h)in formula (54) and formula (55) is denoted by r_(hmax).

$\begin{matrix}{{L_{P} \geq \frac{2\pi \; r_{h}}{\sqrt{\frac{\alpha_{S}}{\alpha_{km}L_{span}}( {\frac{\alpha_{S}}{\alpha_{km}L_{span}} + 2} )}}} = {\frac{2\pi \; \alpha_{km}L_{span}}{\sqrt{\alpha_{S}( {\alpha_{S} + {2\alpha_{km}L_{span}}} )}}r_{h}}} & (54) \\{\mspace{79mu} {{R \geq {\frac{( {\frac{\alpha_{S}}{\alpha_{km}L_{span}} + 1} )^{2}}{( {\frac{\alpha_{S}}{\alpha_{km}L_{span}} + 1} )^{2} - 1}r_{h}}} = {\frac{( {\alpha_{S} + {\alpha_{km}L_{span}}} )^{2}}{\alpha_{S}( {\alpha_{S} + {2\alpha_{km}L_{span}}} )}r_{h}}}} & (55)\end{matrix}$

The right sides of the above formulae (54) and (55) monotonicallyincrease against r_(h) and α_(km). For this reason, we should considermaxima in the cable, for r_(h) and α_(km). This can decrease a minimumthat can be taken by the radius of curvature R of the optical fiber, asα_(km) becomes smaller. In addition, from the above formulae (46a)-(47d)and others, the crosstalk becomes smaller or restrictions on theparameters such as κ and Λ in association with the crosstalk can berelaxed.

Accordingly, α_(km) is preferably, at least, not more than 0.19 dB/km,more preferably not more than 0.18 dB/km, still more preferably not morethan 0.17 dB/km, much more preferably not more than 0.16 dB/km, andfurther much more preferably not more than 0.15 dB/km.

When L_(span) is 80 km as a general value, L_(P) and R preferablysatisfy at least the relations of formulae (56a) and (56b) below.

$\begin{matrix}{{L_{P} \geq {\frac{2{\pi \cdot 0.19 \cdot 80}}{\sqrt{\alpha_{S}( {\alpha_{S} + {2 \cdot 0.19 \cdot 80}} )}}r_{h}}} = {\frac{2{\pi \; \cdot 15.2}}{\sqrt{\alpha_{S}( {\alpha_{S} + {2 \cdot 15.2}} )}}r_{h}}} & ( {56a} ) \\{{R \geq {\frac{( {\alpha_{S} + {0.19 \cdot 80}} )^{2}}{\alpha_{S}( {\alpha_{S} + {2 \cdot 0.19 \cdot 80}} )}r_{h}}} = {\frac{( {\alpha_{S} + 15.2} )^{2}}{\alpha_{S}( {\alpha_{S} + {2 \cdot 15.2}} )}r_{h}}} & ( {56b} )\end{matrix}$

Under the condition of L_(span)=80 km, L_(P) and R more preferablysatisfy the relations of formulae (57a) and (57b) below.

$\begin{matrix}{{L_{P} \geq {\frac{2{\pi \cdot 0.18 \cdot 80}}{\sqrt{\alpha_{S}( {\alpha_{S} + {2 \cdot 0.18 \cdot 80}} )}}r_{h}}} = {\frac{2{\pi \; \cdot 14.4}}{\sqrt{\alpha_{S}( {\alpha_{S} + {2 \cdot 14.4}} )}}r_{h}}} & ( {57a} ) \\{{R \geq {\frac{( {\alpha_{S} + {0.18 \cdot 80}} )^{2}}{\alpha_{S}( {\alpha_{S} + {2 \cdot 0.18 \cdot 80}} )}r_{h}}} = {\frac{( {\alpha_{S} + 14.4} )^{2}}{\alpha_{S}( {\alpha_{S} + {2 \cdot 14.4}} )}r_{h}}} & ( {57b} )\end{matrix}$

Under the condition of L_(span)=80 km, L_(P) and R more preferablysatisfy the relations of formulae (58a) and (58b) below.

$\begin{matrix}{{L_{P} \geq {\frac{2{\pi \cdot 0.17 \cdot 80}}{\sqrt{\alpha_{S}( {\alpha_{S} + {2 \cdot 0.17 \cdot 80}} )}}r_{h}}} = {\frac{2{\pi \; \cdot 13.6}}{\sqrt{\alpha_{S}( {\alpha_{S} + {2 \cdot 13.6}} )}}r_{h}}} & ( {58a} ) \\{{R \geq {\frac{( {\alpha_{S} + {0.17 \cdot 80}} )^{2}}{\alpha_{S}( {\alpha_{S} + {2 \cdot 0.17 \cdot 80}} )}r_{h}}} = {\frac{( {\alpha_{S} + 13.6} )^{2}}{\alpha_{S}( {\alpha_{S} + {2 \cdot 13.6}} )}r_{h}}} & ( {58b} )\end{matrix}$

Under the condition of L_(span)=80 km, L_(P) and R more preferablysatisfy the relations of formulae (59a) and (59b) below.

$\begin{matrix}{{L_{p} \geq {\frac{2{\pi \cdot 0.16 \cdot 80}}{\sqrt{\alpha_{S}( {\alpha_{S} + {2 \cdot 0 \cdot 16 \cdot 80}} )}}r_{h}}} = {\frac{2{\pi \cdot 12.8}}{\sqrt{\alpha_{S}( {\alpha_{S} + {2 \cdot 12.8}} )}}r_{h}}} & ( {59a} ) \\{{R \geq {\frac{( {\alpha_{S} + {0.16 \cdot 80}} )^{2}}{\alpha_{S}( {\alpha_{S} + {2 \cdot 0.16 \cdot 80}} )}r_{h}}} = {\frac{( {\alpha_{S} + 12.8} )^{2}}{\alpha_{S}( {\alpha_{S} + {2 \cdot 12.8}} )}r_{h}}} & ( {59b} )\end{matrix}$

Under the condition of L_(span)=80 km, L_(P) and R more preferablysatisfy the relations of formulae (60a) and (60b) below.

$\begin{matrix}{{L_{p} \geq {\frac{2{\pi \cdot 0.15 \cdot 80}}{\sqrt{\alpha_{S}( {\alpha_{S} + {2 \cdot 0.15 \cdot 80}} )}}r_{h}}} = {\frac{2{\pi \cdot 12.0}}{\sqrt{\alpha_{S}( {\alpha_{S} + {2 \cdot 12.0}} )}}r_{h}}} & ( {60a} ) \\{{R \geq {\frac{( {\alpha_{S} + {0.15 \cdot 80}} )^{2}}{\alpha_{S}( {\alpha_{S} + {2 \cdot 0.15 \cdot 80}} )}r_{h}}} = {\frac{( {\alpha_{S} + 12.0} )^{2}}{\alpha_{S}( {\alpha_{S} + {2 \cdot 12.0}} )}r_{h}}} & ( {60b} )\end{matrix}$

Here, α_(S) is preferably not more than 0.5 dB/span as a maximum, morepreferably not more than 0.3 dB/span, and still more preferably not morethan 0.1 dB/span.

FIG. 11 shows relations of the pitch L_(P) of the helix and the radiusof curvature R, from the above formula (49). In a thin cable, thedistances from the cable center to optical fibers on the cable crosssection can be as short as about 2 mm. In terms of manufacture of thecable, the pitch L_(P) of the helix on the occasion of housing theoptical fibers in the helix shape in the cable is preferably, at least,not less than 200 mm and more preferably not less than 300 mm. In FIG.11, graph G1101 shows the relation between L_(P) and R with the radiusof the helix being set at 2 mm, graph G1102 the relation between L_(P)and R with the radius of the helix being set at 3 mm, graph G1103 therelation between L_(P) and R with the radius of the helix being set at 4mm, graph G1104 the relation between L_(P) and R with the radius of thehelix being set at 5 mm, graph G1105 the relation between L_(P) and Rwith the radius of the helix being set at 6 mm, graph G1106 the relationbetween L_(P) and R with the radius of the helix being set at 7 mm,graph G1107 the relation between L_(P) and R with the radius of thehelix being set at 8 mm, graph G1108 the relation between L_(P) and Rwith the radius of the helix being set at 9 mm, graph G1109 the relationbetween L_(P) and R with the radius of the helix being set at 10 mm,graph G1110 the relation between L_(P) and R with the radius of thehelix being set at 11 mm, and graph G1111 the relation between L_(P) andR with the radius of the helix being set at 12 mm.

From these, when R and r_(h) are expressed in millimeter unit, therelation between the radius of curvature R and r_(h) of fiber preferablysatisfies at least formula (61) below.

$\begin{matrix}{R \geq \frac{r_{h}^{2} + ( \frac{200}{2\pi} )^{2}}{r_{h}}} & (61)\end{matrix}$

Furthermore, the relation between the radius of curvature R and r_(h) offiber more preferably satisfies at least formula (62) below.

$\begin{matrix}{R \geq \frac{r_{h}^{2} + ( \frac{300}{2\pi} )^{2}}{r_{h}}} & (62)\end{matrix}$

From the viewpoint of crosstalk, the radius of curvature R of fiberneeds to satisfy the above formula (47d). For this reason, from theabove formulae (46d) and (49), the pitch L_(P) of the helix needs tosatisfy formula (63a) below. Alternatively, from the above formulae(47d) and (49), the pitch L_(P) of the helix needs to satisfy formula(63b) below. It is noted that the smallest value of r_(h) in formula(63a) and formula (63b) is represented by r_(hmin).

$\begin{matrix}{L_{P} \leq {2\pi \sqrt[\;]{{{\frac{1}{2}\frac{\beta}{\kappa^{2}}D_{nm}\frac{{XT}_{S}}{L_{F}}r_{h}} - r_{h}^{2}}}}} & ( {63a} ) \\{L_{P} \leq {2\pi \sqrt[\;]{{{\frac{1}{12}\frac{\beta}{\kappa^{2}}\Lambda \frac{{XT}_{S}}{L_{F}}r_{h}} - r_{h}^{2}}}}} & ( {63b} )\end{matrix}$

The above described the case where the fibers were incorporated in thehelix shape in the cable and where the center axis of rotation of thehelix was at the center of the cable cross section, but the center axisof rotation of the helix does not always have to be at the center of thecable cross section, and a cable may be arranged in such a configurationthat there are a plurality of center axes of rotation of differenthelices in the cable.

Second Embodiment

In the second embodiment, which will be described below, a relation offormula (64) below also holds, where n_(eff-m) is the effective index ofcore m among a plurality of cores in the multi-core fiber 100A,n_(eqeff-nm) the equivalent index of the effective index of core m onthe basis of core n, D_(nm) the core distance (intercentral distance)between core n and core m, and φ_(nm) (rad) the angle between thestraight line mn and the straight line agreeing with the bending radiusdirection of the multi-core fiber 100A. The straight line mn means aline connecting the center of core n and the center of core m on thecross section of the multi-core fiber 100A perpendicular to thepredetermined axis AX.

$\begin{matrix}{n_{{eqeff}\text{-}{nm}} = {n_{{eff}\text{-}m}\{ {1 + \frac{D_{nm}{\sin ( \phi_{nm} )}}{R}} \}}} & (64)\end{matrix}$

When the above formula (64) is considered with the propagation constantsin place of the effective indices, formula (65) below is obtainedbecause β=(2π/λ)n_(eff) (where λ is the wavelength and n_(eff) theeffective index).

$\begin{matrix}{\beta_{{eq}\text{-}{nm}} = {\beta_{m}\{ {1 + \frac{D_{nm}{\sin ( \phi_{nm} )}}{R}} \}}} & (65)\end{matrix}$

In the above formula, β_(m) is the propagation constant of core m andβ_(eq-nm) the propagation constant of core m taking account of theequivalent index on the basis of core n.

In this case, the difference Δβ_(nm) between β_(eq-nm) and β_(eq-nn)(which is not the relative refractive-index difference) is given byformula (66) below.

$\begin{matrix}{{\Delta \; \beta_{nm}} = {{\beta_{{eq}\text{-}{nm}} - \beta_{{eq}\text{-}{nn}}} = {{{\beta_{m}\{ {1 + \frac{D_{nm}{\sin ( \phi_{nm} )}}{R}} \}} - \beta_{n}} = {{\beta_{m}\frac{D_{n\; m}{\sin ( \phi_{n\; m} )}}{R}} + ( {\beta_{m} - \beta_{n}} )}}}} & (66)\end{matrix}$

It is considered that the inter-core crosstalk decreases as the rate ofΔβ_(nm) being close to 0 becomes smaller along the longitudinaldirection of the multi-core fiber. When the parameter R=30 mm ispermitted, it is not easy to prevent the difference Δβ_(nm) from alwaysbecoming 0, with the core distance D_(nm)=30 μm between core n and corem. Specifically, the reason for it is that it becomes necessary to makethe difference between the propagation constant βP_(n) and thepropagation constant β_(m) such that the relative refractive-indexdifference Δ_(eff) between effective indices exceeds 0.1%, as shown inFIG. 4B.

There are zeros of Δβ_(nm) along the longitudinal direction of themulti-core fiber, and it is considered that it is desirable that theslope of Δβ_(nm) at each zero be steep and the frequency of appearanceof zeros be low. Particularly, it is important that the slope of Δβ_(nm)at each zero be steep.

In order to control the slope of Δβ_(nm) at each zero and the frequencyof appearance of zeros, it is preferable to provide the optical fiberwith a properly controlled elastic twist or plastic twist. However,without need for intentional provision of the twist, the optical fiberis elastically or plastically twisted at random in the longitudinaldirection.

The below will describe the result of simulation associated with theabove discussion.

FIG. 7 is a graph showing variation in inter-core crosstalk (which isreferred to simply as “crosstalk” in FIG. 7) along the longitudinaldirection of a multi-core fiber with two cores (which will be referredto hereinafter as a 2-core fiber), which is specifically variationappearing along the longitudinal direction of the 2-core fiber and withincidence of light with the optical intensity I₁=1 into one of the twocores, in the optical intensity I₂ of the other core. When theinter-core crosstalk is defined as (intensity of a certain non-incidentcore)/(total of intensities of all cores), the graph of FIG. 7 can besaid to be a graph of variation in crosstalk along the longitudinaldirection of the 2-core fiber. In this 2-core fiber, the two cores havethe refractive-index profile of the same structure, each core Δ to thecladding region is 0.34%, each core diameter 9 μm, and the core distanceD 40 μm. The 2-core fiber is provided with a bend of the radius of 300mm throughout the entire length. Furthermore, the fiber is given a twistalong the longitudinal direction of the 2-core fiber (unidirectionalrotation about the axis of the 2-core fiber). This twist provides the2-core fiber with one rotation per 10 m. Namely, when z stands for thelongitudinal position of the 2-core fiber, there are two zeros ofΔβ_(nm)(z) per 10 m. In FIG. 7, steep changes of crosstalk existing atequal intervals and at a rate of two per 10 m are zeros of Δβ_(nm)(z).

The above-described simulation was to calculate the variation ofinter-core crosstalk, and then expressions to express the behavior ofcrosstalk more simply will be established below.

The inverse of the slope given by formula (67a) below, at an arbitraryzero z of Δβ_(nm) (z) can be used as an index indicating how longΔβ_(nm)(z) is located near 0 in passing the zero z. Then a crosstalkamount χ between cores at the arbitrary zero is expressed with an indexof formula (67b) below and the crosstalk amount χ between cores isconsidered to decrease as the value of this parameter 1 becomes smaller.

$\begin{matrix}{\frac{\;}{z}\Delta \; {\beta_{nm}(z)}} & ( {67a} ) \\{l = {\frac{1}{{\frac{\;}{z}{{\Delta\beta}_{nm}(z)}}_{{{\Delta\beta}_{nm}{(z)}} = 0}}}} & ( {67b} )\end{matrix}$

Furthermore, it is considered that significant inter-core crosstalkoccurs near extremes of zeros z only. When the aforementioned formula(10) and formula (11) are considered herein, F=1 and L=(π/2)·(1/κ) fromthe relation of formula (68a) below. When coupling between two cores isconsidered, in the case where F=1 and L=(π/2)·(1/κ) and where light withthe intensity I₁=1 is incident into one core 1, formula (68b) belowrepresents the intensity I₂ at the longitudinal position z of the 2-corefiber, in the other core 2.

$\begin{matrix}{\psi = {{\Delta \; {\beta_{12}/2}} = 0}} & ( {68a} ) \\{I_{2} = {\sin^{2}( {\frac{\kappa}{2\pi}z} )}} & ( {68b} )\end{matrix}$

Further supposing I₁>>I₂, z in the above formula (68b) can be assumed tobe near 0, in the vicinity of each zero of Δβ_(nm)(z). Then theintensity I₂ can be approximated by formula (69) below.

$\begin{matrix}{I_{2} \approx {( \frac{\kappa}{2\pi} )^{2}z^{2}}} & (69)\end{matrix}$

Furthermore, with consideration including the fact that the respectivevalues of F and L gradually change with deviation from a zero ofΔβ_(nm)(z), the crosstalk amount χ between cores near an arbitrary zeroof Δβ_(nm)(z) is considered to be expressed by formula (70) below,eventually in the case of I₁>>I₂.

$\begin{matrix}{\chi = {( \frac{\kappa_{nm}}{2\pi} )^{2}\alpha {\frac{1}{{\frac{\;}{z}\Delta \; {\beta_{nm}(z)}}_{{{\Delta\beta}_{nm}{(z)}} = 0}}}}} & (70)\end{matrix}$

In the above formula, α is a coefficient to relate above formula (67b)to above formula (69).

The inter-core crosstalk amount χ will be determined below for somecases.

φ_(nm) is a function of z among the parameters in formula (66) above andlet us consider a case where the relation of formula (71) below holds(where γ_(c)≠0).

φ_(nm)(z)=γ_(c) z  (71)

In this case, where the longitudinal position z of the 2-core fiber isgiven by formula (72a) below, Δβ_(nm)(z)=0; the relation represented byformula (72b) below holds at any point and the inter-core crosstalkamount χ at any point is given by formula (72c) below.

$\begin{matrix}{z = \{ {\begin{matrix}{\frac{1}{\gamma_{c}}\{ {{a\; {\sin ( {\frac{R}{D_{n,m}}\frac{\beta_{n} - \beta_{m}}{\beta_{m}}} )}} + {2\pi \; k}} \}} \\{\frac{1}{\gamma_{c}}\{ {\pi - {a\; {\sin ( {\frac{R}{D_{n,m}}\frac{\beta_{n} - \beta_{m}}{\beta_{m}}} )}} + {2\pi \; k}} \}}\end{matrix}( {{{where}\mspace{14mu} k\mspace{14mu} {is}\mspace{14mu} {an}\mspace{14mu} {integer}};{a\mspace{14mu} {range}\mspace{14mu} {of}\mspace{14mu} a\; {\sin (x)}\mspace{14mu} {is}\mspace{14mu} {( {{- \frac{\pi}{2}},\frac{\pi}{2}} \rbrack.}}} )} } & ( {72a} ) \\{{{\frac{\;}{z}\Delta \; {\beta_{nm}(z)}}} = {\beta_{m}{\gamma_{c}}\sqrt{( \frac{D_{nm}}{R} )^{2} - ( \frac{\beta_{n} - \beta_{m}}{\beta_{m}} )^{2}}}} & ( {72b} ) \\{\chi = {{\alpha ( \frac{\kappa_{nm}}{2\pi} )}^{2}\frac{1}{\beta_{m}}\frac{1}{\gamma_{c}}\frac{1}{\sqrt{{( \frac{D_{nm}}{R} )^{2} - ( \frac{\beta_{n} - \beta_{m}}{\beta_{m}} )^{2}}\;}}}} & ( {72c} )\end{matrix}$

In the case of a relation represented by formula (73a) below (whereγ_(a)≧π and γ_(f)>0), a relation represented by formula (73c) belowholds at the longitudinal position z of the 2-core fiber whereΔβ_(nm)(z)=0 (formula (73b) below), and the inter-core crosstalk amountχ is given by formula (73d) below.

$\begin{matrix}{{\phi_{n,m}(z)} = {\gamma_{a}{\sin ( {\gamma_{f}z} )}}} & ( {73a} ) \\{z = \{ \begin{matrix}{\frac{1}{\gamma_{f}}\{ {{a\; {\sin ( {\frac{1}{\gamma_{a}}\{ {{a\; {\sin ( {\frac{R}{D_{nm}}\frac{\beta_{n} - \beta_{m}}{\beta_{m}}} )}} + {2\pi \; k_{1}}} \}} )}} + {2\pi \; k_{3}}} \}} \\{\frac{1}{\gamma_{f}}\{ {{a\; {\sin ( {\frac{1}{\gamma_{a}}\{ {\pi - \; {a\; {\sin ( {\frac{R}{D_{nm}}\frac{\beta_{n} - \beta_{m}}{\beta_{m}}} )}} + {2\pi \; k_{2}}} \}} )}} + {2\pi \; k_{3}}} \}} \\{\frac{1}{\gamma_{f}}\{ {\pi - \; {a\; {\sin ( {\frac{1}{\gamma_{a}}\{ {{a\; {\sin ( {\frac{R}{D_{nm}}\frac{\beta_{n} - \beta_{m}}{\beta_{m}}} )}} + {2\pi \; k_{1}}} \}} )}} + {2\pi \; k_{3}}} \}} \\{\frac{1}{\gamma_{f}}\{ {\pi - \; {a\; {\sin ( {\frac{1}{\gamma_{a}}\{ {\pi - \; {a\; {\sin ( {\frac{R}{D_{nm}}\frac{\beta_{n} - \beta_{m}}{\beta_{m}}} )}} + {2\pi \; k_{2}}} \}} )}} + {2\pi \; k_{3}}} \}}\end{matrix} } & ( {73b} )\end{matrix}$

(where each of k₁ and k₂ is an integer in a range satisfying the domainof the arcsine function in the formula.)

$\begin{matrix}{{{\frac{\;}{z}\Delta \; {\beta_{n,m}(z)}}} = \{ \begin{matrix}\begin{matrix}{\beta_{m}\gamma_{f}\sqrt{( \frac{D_{nm}}{R} )^{2} - ( \frac{\beta_{n} - \beta_{m}}{\beta_{m}} )^{2}}} \\\sqrt{\gamma_{a}^{2} - \{ {{a\; {\sin ( {\frac{R}{D_{nm}}\frac{\beta_{n} - \beta_{m}}{\beta_{m}}} )}} + {2\pi \; k_{1}}} \}^{2}}\end{matrix} \\\begin{matrix}{\beta_{m}\gamma_{f}\sqrt{( \frac{D_{nm}}{R} )^{2} - ( \frac{\beta_{n} - \beta_{m}}{\beta_{m}} )^{2}}} \\\sqrt{\gamma_{a}^{2} - \{ {\pi - {a\; {\sin ( {\frac{R}{D_{nm}}\frac{\beta_{n} - \beta_{m}}{\beta_{m}}} )}} + {2\pi \; k_{2}}} \}^{2}}\end{matrix}\end{matrix} } & ( {73c} ) \\{\chi = \{ \begin{matrix}\begin{matrix}{{\alpha ( \frac{\kappa_{nm}}{2\pi} )}^{2}\frac{1}{\beta_{m}}\frac{1}{\gamma_{f}}\frac{1}{\sqrt{( \frac{D_{nm}}{R} )^{2} - ( \frac{\beta_{n} - \beta_{m}}{\beta_{m}} )^{2}}}} \\\frac{1}{\sqrt{\gamma_{a}^{2} - \{ {{a\; {\sin ( {\frac{R}{D_{nm}}\frac{\beta_{n} - \beta_{m}}{\beta_{m}}} )}} + {2\pi \; k_{1}}} \}^{2}}}\end{matrix} \\\begin{matrix}{{\alpha ( \frac{\kappa_{nm}}{2\pi} )}^{2}\frac{1}{\beta_{m}}\frac{1}{\gamma_{f}}\frac{1}{\sqrt{( \frac{D_{nm}}{R} )^{2} - ( \frac{\beta_{n} - \beta_{m}}{\beta_{m}} )^{2}}}} \\\frac{1}{\sqrt{\gamma_{a}^{2} - \{ {\pi - {a\; {\sin ( {\frac{R}{D_{nm}}\frac{\beta_{n} - \beta_{m}}{\beta_{m}}} )}} + {2\pi \; k_{2}}} \}^{2}}}\end{matrix}\end{matrix} } & ( {73d} )\end{matrix}$

(where each of k₁ and k₂ is an integer in a range satisfying the domainof the arcsine function in the formulae.)

From the above discussion, in order to decrease the inter-core crosstalkamount χ in the 2-core fiber, it is necessary to increase the coredistance D_(nm) between the two cores n and m, to decrease the parameterR (the radius of curvature for the 2-core fiber), or to decrease thedifference between the propagation constant β_(n) of the core n and thepropagation constant β_(m) of the core m (i.e., to decrease thedifference between n_(eff-n) and n_(eff-m)). Particularly, an increaseof the core distance D_(nm) between core n and core m leads naturally toa decrease of the coupling coefficient κ between cores, so as to achievea significant effect of reduction in crosstalk between cores.Furthermore, the inter-core crosstalk amount χ can also be decreased byincreasing the parameters γ_(c) and γ_(f).

As also seen from the above description, it is desirable to keepn_(eff-n)=n_(eff-m) in terms of the inter-core crosstalk amount as well,and it is also readily feasible in terms of manufacture to achieve themulti-core fiber 100A because it can be manufactured in the same corestructure. In the description hereinafter, therefore, the case ofn_(eff-n)=n_(eff-m) will be discussed.

When n_(eff-n)=n_(eff-m), the above formula (72c) can be expressed byformula (74a) below and the above formula (73 d) by formula (74b) below.

$\begin{matrix}{\chi = {{\alpha ( \frac{\kappa_{nm}}{2\; \pi} )}^{2}\frac{1}{\beta_{m}}\frac{1}{\gamma_{c}}\frac{R}{D_{nm}}}} & ( {74\; a} ) \\{{\chi = {{\alpha ( \frac{\kappa_{nm}}{2\; \pi} )}^{2}\frac{1}{\beta_{m}}\frac{1}{\gamma_{f}}\frac{R}{D_{nm}}\frac{1}{\sqrt{\gamma_{a}^{2} - ( {\pi \; k} )^{2\;}}}}}( {{{{where}\mspace{14mu} k\mspace{14mu} {is}\mspace{14mu} {an}\mspace{14mu} {integer}\mspace{14mu} {satisfying}} - \frac{\gamma_{a}}{\pi}} \leq k \leq {\frac{\gamma_{a}}{\pi}.}} )} & ( {74\; b} )\end{matrix}$

For simplicity, let us now consider the case where the relationrepresented by the above formula (71) holds, i.e., the case of the aboveformula (74a).

A simulation was carried out in the same manner as the simulationobtaining the result of FIG. 7, and for a 2-core fiber with core 1 andcore 2 of the same structure, we obtained a relation between thecrosstalk amount χ between cores with α=1 and an average of crosstalkbetween the first zero and the second zero of Δβ₁₂(z), i.e., therelation between the crosstalk amount χ and the crosstalk variationamount at the initial zero of Δβ₁₂(z). In this case, FIG. 8 shows theresults calculated for all combinations of the wavelength of 1.55 μm,core Δ of 0.34% and 0.4%, the parameter R of 60 mm, 120 mm, 180 mm, 240mm, and 300 mm, and the core distance D₁₂ between core 1 and core 2 of35 μm and 40 μm. As seen from this FIG. 8, with parameter α=19.09373,the crosstalk amount χ becomes the crosstalk variation amount at theinitial zero of Δβ₁₂(z).

Incidentally, the crosstalk variation amount at the initial zero has theorderly nature of the law as described above. It seems, however, thatthere is little nature of a law for the crosstalk variation amounts atthe second and subsequent zeros in view of FIG. 7. This is because thephase of light undergoing crosstalk at a certain zero from core 1 tocore 2 in the 2-core fiber does not match the phase of light undergoingcrosstalk from core 1 to core 2 at a next zero. As a result, thecrosstalk at the second and subsequent zeros stochastically varieswithin a certain range.

Specifically, when consideration is made based on variation in electricfield instead of intensity, it is considered that, where at the secondor subsequent zero, θ represents a phase shift between light in core 2and light undergoing crosstalk from core 1 to core 2, approximatevariation in electric field given by formula (75a) below occurs, thoughdepending upon the amplitudes in the core 1 and core 2 before thepertinent zero. For this reason, the amplitude of the electric field atthe exit end of this 2-core fiber is considered to take stochasticvalues given by a probability distribution of a normal distribution withan average μ, of the parameter given by formula (75b) below and with afixed variance σ², by the central limit theorem. Since the variance offormula (75a) is χ/2 on the assumption that the phase shift θ isuniformly random, when zeros in the overall fiber length are representedby n_(zero), the above variance σ² is given by (χ/2)n_(zer0), obtainingformula (75c) below. When core 1 and core 2 satisfy the relation of theabove formula (71) (n=1 and m=2 in formula (71)), n_(zero)=γ_(c)L_(F)/π,and thus σ is given by formula (75d) below.

$\begin{matrix}{\sqrt{\chi}\cos \; \theta} & ( {75\; a} ) \\\sqrt{\chi} & ( {75\; b} ) \\{\sigma = \sqrt{\frac{\chi}{2}n_{zero}}} & ( {75\; c} ) \\{\sigma = {\frac{\kappa_{nm}}{2\; \pi}\sqrt{\frac{\alpha}{2\; \pi}\frac{1}{\beta_{m}}\frac{r}{D_{nm}}}\sqrt{L_{F}}}} & ( {75\; d} )\end{matrix}$

Namely, when core 1 and core 2 satisfy the relation of the above formula(71) (n=1 and m=2 in formula (71)), if I₁=1 and I₂=0 at the entrance endof the 2-core fiber, the amplitude of the electric field in core 2 atthe exit end of this 2-core fiber is given by formula (76a) below, andtakes stochastic values according to a probability distribution of anormal distribution represented by formula (76b) below.

$\begin{matrix}{\mu = {\frac{\kappa_{nm}}{2\; \pi}\sqrt{\alpha \frac{1}{\beta_{m}}\frac{R}{D_{nm}}}\sqrt{\frac{1}{\gamma_{c}}}}} & ( {76\; a} ) \\{\sigma = {\frac{\kappa_{nm}}{2\; \pi}\sqrt{\frac{\alpha}{2\; \pi}\frac{1}{\beta_{m}}\frac{R}{D_{nm}}}\sqrt{L_{F}}}} & ( {76\; b} )\end{matrix}$

When P_(XT) is defined as a probability that the inter-core crosstalk atthe exit end of the 2-core fiber is not more than XT, the probabilityP_(XT) is represented by formula (77) below.

$\begin{matrix}\begin{matrix}{P_{XT} = {\int_{0}^{\sqrt{XT}}\{ {{\frac{1}{\sqrt{2\; \pi}\sigma}{\exp ( {- \frac{( {x - u} )}{2\; \sigma^{2}}} )}} +} }} \\{ {\frac{1}{\sqrt{{2\; \pi}\;}\sigma}{\exp ( {- \frac{( {x + \mu} )}{2\; \sigma^{2}}} )}} \} \ {x}} \\{= {\frac{1}{2}\{ {{{erf}( \frac{\sqrt{XT} - \mu}{\sigma \sqrt{2}} )} + {{erf}( \frac{\sqrt{XT} + \mu}{\sigma \sqrt{2}} )}} \}}} \\{= {\frac{1}{2}\{ {{{erf}( {{\frac{2\; \pi \sqrt{\pi \; \beta_{m}D_{nm}}}{\kappa_{nm}\sqrt{\alpha \; {RL}_{F}}}\sqrt{XT}} - \sqrt{\frac{\pi}{\gamma_{c}L_{F}}}} )} +} }} \\ {{erf}( {{\frac{2\; \pi \sqrt{\pi \; \beta_{m}D_{nm}}}{\kappa_{nm}\sqrt{\alpha \; {RL}_{F}}}\sqrt{XT}} + \sqrt{\frac{\pi}{\gamma_{c}L_{F}}}} )} \}\end{matrix} & (77)\end{matrix}$

(where erf(x) is an error function.)

Since L_(F) is at least several to several ten km or more in ordinaryuse, it is considered that the condition represented by formula (78a)below is satisfied. For satisfying formula (78a) or formula (78b) belowwith L_(F) being relatively short, formula (78c) below needs to be true.γ_(c) needs to increase with decrease of L_(F), and γ_(c)≧2π (rad/m)with L_(F)=5000 m and γ_(c)≧10π (rad/m) with L_(F)=1000 m, which can berealized readily.

$\begin{matrix}{\sqrt{\frac{\pi}{\gamma_{c}L_{F}}}{\operatorname{<<}1}} & ( {78\; a} ) \\{\sqrt{\frac{\pi}{\gamma_{c}L_{F}}} \leq 0.01} & ( {78\; b} ) \\{\gamma_{c} \geq {\pi \frac{10^{4}}{L_{F}}}} & ( {78\; c} )\end{matrix}$

In order to provide the multi-core fiber 100A with the desired twistamount γ_(c), the multi-core fiber 100A is rotated along arrow S aroundthe axis AX (direction around the axis), as shown in FIG. 12A. In thiscase, the twist given to the multi-core fiber 100A may be an elastic orplastic twist and this twist varies along the longitudinal direction ofthe fiber on the basis of the direction of the radius of the curvaturegiven to the multi-core fiber 100A. Specifically, S_(x)-S_(y)coordinates defining the cross section of the multi-core fiber 100Arotate along the longitudinal direction of the multi-core fiber 100A, asshown in FIG. 12B, whereby the multi-core fiber 100A is given the twistat the predetermined pitch.

When the above formula (78a) is met, a relation of formula (79) belowcan be considered to hold for the probability P_(XT) and thus theprobability of crosstalk becoming not more than the fixed valueincreases.

$\begin{matrix}{P_{XT} \approx {{erf}( {\frac{2\; \pi}{\kappa_{nm}}\sqrt{\frac{\pi \; \beta_{m}D_{nm}}{\alpha \; {RL}_{F}}}\sqrt{XT}} )}} & (79)\end{matrix}$

Since the function erf(x) is a monotonically increasing function, itneeds to satisfy a relation represented by formula (80a) below, whenP_(XT)≧0.9999. Formula (80b) is an expression obtained by expandingformula (80a) so as to obtain a conditional expression to be satisfiedby the parameter R in formula (80a).

$\begin{matrix}{{\frac{2\; \pi}{\kappa_{nm}}\sqrt{\frac{\pi \; \beta_{m}D_{nm}}{\alpha \; {RL}_{F}}}\sqrt{XT}} \geq {{erf}^{- 1}(0.9999)}} & ( {80\; a} ) \\{R \leq {\frac{1}{\{ {{erf}^{- 1}(0.9999)} \}^{2}}( \frac{2\; \pi}{\kappa_{nm}} )^{2}\frac{\pi \; D_{n,m}\beta_{m}{XT}}{\alpha \; L_{F}}}} & ( {80\; b} )\end{matrix}$

When a parameter R_(th) (radius of curvature) is defined as in formula(81) below, based on the above formula (80b), for example, in the caseof the 2-core fiber with core Δ of 0.4%, the core diameter of 9 μm, thecore distance of 40 μm, L_(F)=100 km, and XT=0.001, R_(th) can berepresented by formula (81) below and R_(th) is 14.1 mm for light of thewavelength of 1.55 μm. At this time, the crosstalk after propagationthrough 100 km is not more than −30 dB with the probability of not lessthan 99.99%. It is, however, found by simulation that the bending lossof not less than 10 dB/km occurs, and it is thus seen that it isinfeasible to achieve long-haul transmission.

$\begin{matrix}{R_{th} = {\frac{1}{\{ {{erf}^{- 1}(0.9999)} \}^{2}}( \frac{2\; \pi}{\kappa_{nm}} )^{2}\frac{\pi \; D_{nm}\beta_{m}{XT}}{\alpha \; L_{F}}}} & (81)\end{matrix}$

FIG. 13 shows κ, R_(th) (mm), the bending loss (dB/km) with thecurvature radius R_(th), and the core diameter (μm) against core Δ,under the conditions of the core distance of 40 μm, the core diameteradjusted to one in the case of the cable cutoff wavelength of 1.53 μm,L_(F)=100 krn, XT=0.001, and propagation of light of the wavelength of1.55 μm. In FIG. 13, graph G1110 indicates κ, graph G1120 the curvatureradius R_(th) (mm), graph G1130 the bending loss (dB/km) with thecurvature radius R_(th), and graph G1140 the core diameter (μm). In FIG.13, the bending loss is not plotted in the range where the core Δ islarger than 0.38%, because the values thereof are very small.

As seen from this FIG. 13, in order to control a loss increase due tobending after 100 km propagation to 1 dB, it is necessary to keep thecore Δ not less than 0.373% with the bending loss being not more than0.01 dB/km; in order to control the loss increase due to bending after100 km propagation to 0.1 dB or less, it is necessary to keep the core Δnot less than 0.378% with the bending loss being not more than 0.001dB/km. When the cable cutoff wavelength is shorter than 1530 nm, thecore Δ needs to be higher.

As described above, where the cable cutoff wavelength is 1530 nm, it ispreferable in order to realize a low bending loss even with a bend ofnot more than the aforementioned radius R_(th) that on the cross sectionperpendicular to the predetermined axis, the core distance D in themulti-core fiber 100A be not less than 40 μm and the relativerefractive-index difference Δ of each of the cores 110A1, 110B1-B3,110C1-110C3 to the cladding region 120 be not less than 0.373%.

Since the present invention provides the optical fiber cable with thestructure in which the jacket or the like covering the multi-core fibersholds them in the state in which each multi-core fiber with a pluralityof cores is provided with a bend of an appropriate radius of curvature,the crosstalk between cores can be controlled at a low level even insituations where signal light propagates between repeaters, betweenstations, or between a terminal and a station between which the opticalfiber cable is laid.

1. An optical fiber cable incorporating a multi-core fiber comprising aplurality of cores each extending along a predetermined axis, and acladding region integrally surrounding the plurality of cores, saidoptical fiber cable comprising a structure to provide the multi-corefiber with a bend of the smallest value of radii of curvature R_(th)given by the following expression:$R_{th} = {\frac{1}{2}\frac{\beta}{\kappa^{2}}D_{nm}\frac{{XT}_{S}}{L_{F}}}$where D_(nm) is an intercentral distance between core n and core m inthe multi-core fiber, L_(F) a fiber length of the multi-core fibercorresponding to a length between repeater/regenerators in laying theoptical fiber cable, β a propagation constant of each core at a firstwavelength, κ a coupling coefficient between adjacent cores at the firstwavelength, and XT_(S) a maximum value permitted as an average of adistribution of crosstalk after propagation of light of the firstwavelength through the fiber length L_(F).
 2. The optical fiber cableaccording to claim 1, wherein the bend providing structure incorporatesthe multi-core fiber in a helix shape in the optical fiber cable,thereby providing the multi-core fiber with the bend of not more than afixed radius of curvature, and wherein the multi-core fiber satisfiesthe following expression:$L_{p} \leq {2\; \pi \sqrt{{{\frac{1}{2}\frac{\beta}{\kappa^{2}}D_{nm}\frac{{XT}_{S}}{L_{F}}r_{h\; \min}} - r_{h\; \min}^{2}}}}$where r_(h) is a radius of the helix, L_(P) a pitch of the helix, andr_(hmin) the smallest r_(h) in the multi-core fiber.
 3. The opticalfiber cable according to claim 1, wherein the maximum value XT_(S)permitted as the average of the distribution of crosstalk after thepropagation of the light of the first wavelength through the fiberlength L_(F)=100 km or more is 0.001.
 4. The optical fiber cableaccording to claim 2, wherein the bend providing structure incorporatesthe multi-core fiber in the helix shape in the optical fiber cable,thereby providing the multi-core fiber with the bend of not more thanthe fixed radius of curvature, and wherein the multi-core fibersatisfies the following expression:$L_{p} \geq {\frac{2\; \pi \; \alpha_{km}L_{span}}{\sqrt{\alpha_{S}( {\alpha_{S} + {2\; \alpha_{km}L_{span}}} )}}r_{h\; \max}}$where r_(h) is the radius of the helix, L_(P) the pitch of the helix,r_(hmax) the largest R_(h) in the multi-core fiber, L_(span) (km) a spanlength, α_(km) (dB/km) a maximum value of transmission losses of therespective cores in the multi-core fiber at a second wavelength, andα_(S) (dB/span) a permissible value per span as a loss increase due toincorporation of the multi-core fiber in the helix shape in the opticalfiber cable.
 5. The optical fiber cable according to claim 4, whereinthe permissible value per span α_(S) as the loss increase due to theincorporation of the multi-core fiber in the helix shape in the opticalfiber cable is not more than 0.5 dB/span.
 6. The optical fiber cableaccording to claim 4, wherein at the wavelength of 1550 nm, thepermissible value per span α_(S) as the loss increase due to theincorporation of the multi-core fiber in the helix shape in the opticalfiber cable is not more than 0.3 dB/span.
 7. The optical fiber cableaccording to claim 4, wherein at the wavelength of 1550 nm, thepermissible value per span α_(S) as the loss increase due to theincorporation of the multi-core fiber in the helix shape in the opticalfiber cable is not more than 0.1 dB/span.
 8. The optical fiber cableaccording to claim 4, wherein at the wavelength of 1550 nm, a value ofthe product (α_(km)·L_(span) km) of the maximum value α_(km) oftransmission losses of the respective cores in the multi-core fiber andthe span length L_(span) is not more than 15.2.
 9. An optical fibercable incorporating a multi-core fiber comprising a plurality of coreseach extending along a predetermined axis, and a cladding regionintegrally surrounding the plurality of cores, said optical fiber cablecomprising a bend providing structure to provide the multi-core fiberwith a bend of a radius of curvature R given by the followingexpression:$R \leq {\frac{1}{12}\frac{\beta}{\kappa^{2}}\Lambda \frac{{XT}_{S}}{L_{F}}}$where Λ is an intercentral distance between adjacent cores in themulti-core fiber, L_(F) a fiber length of the multi-core fibercorresponding to a length between repeater/regenerators in laying theoptical fiber cable, β a propagation constant of each core at a firstwavelength, κ a coupling coefficient between adjacent cores at the firstwavelength, and XT_(S) a maximum value permitted as an average of adistribution of crosstalk after propagation of light of the firstwavelength through the fiber length L_(F).
 10. The optical fiber cableaccording to claim 9, wherein the bend providing structure incorporatesthe multi-core fiber in a helix shape in the optical fiber cable,thereby providing the multi-core fiber with the bend of not more than afixed radius of curvature, and wherein the multi-core fiber satisfiesthe following expression:$L_{p} \leq {2\; \pi \sqrt{{{\frac{1}{12}\frac{\beta}{\kappa^{2}}\Lambda \frac{{XT}_{S}}{L_{F}}r_{h\; \min}} - r_{h\; \min}^{2}}}}$where r_(h) is a radius of the helix, L_(P) a pitch of the helix, andr_(hmin) the smallest r_(h) in the multi-core fiber.
 11. The opticalfiber cable according to claim 9, wherein the maximum value XT_(S)permitted as the average of the distribution of crosstalk after thepropagation of the light of the first wavelength through the fiberlength L_(F)=100 km or more is 0.001.
 12. The optical fiber cableaccording to claim 10, wherein the bend providing structure incorporatesthe multi-core fiber in the helix shape in the optical fiber cable,thereby providing the multi-core fiber with the bend of not more thanthe fixed radius of curvature, and wherein the multi-core fibersatisfies the following expression:$L_{p} \geq {\frac{2\; \pi \; \alpha_{km}L_{span}}{\sqrt{\alpha_{S}( {\alpha_{S} + {2\; \alpha_{km}L_{span}}} )}}r_{h\; \max}}$where r_(h) is the radius of the helix, L_(P) the pitch of the helix,r_(hmax) the largest R_(h) in the multi-core fiber, L_(span) (km) a spanlength, α_(km) (dB/km) a maximum value of transmission losses of therespective cores in the multi-core fiber at a second wavelength, andα_(S) (dB/span) a permissible value per span as a loss increase due toincorporation of the multi-core fiber in the helix shape in the opticalfiber cable.
 13. The optical fiber cable according to claim 12, whereinthe permissible value per span α_(S) as the loss increase due to theincorporation of the multi-core fiber in the helix shape in the opticalfiber cable is not more than 0.5 dB/span.
 14. The optical fiber cableaccording to claim 12, wherein at the wavelength of 1550 nm, thepermissible value per span α_(S) as the loss increase due to theincorporation of the multi-core fiber in the helix shape in the opticalfiber cable is not more than 0.3 dB/span.
 15. The optical fiber cableaccording to claim 12, wherein at the wavelength of 1550 nm, thepermissible value per span α_(S) as the loss increase due to theincorporation of the multi-core fiber in the helix shape in the opticalfiber cable is not more than 0.1 dB/span.
 16. The optical fiber cableaccording to claim 12, wherein at the wavelength of 1550 nm, a value ofthe product (α_(km)·L_(span) km) of the maximum value α_(km) oftransmission losses of the respective cores in the multi-core fiber andthe span length L_(span) is not more than 15.2.
 17. An optical fibercable incorporating a multi-core fiber comprising a plurality of coreseach extending along a predetermined axis, and a cladding regionintegrally surrounding the plurality of cores, said optical fiber cablecomprising a structure to provide the multi-core fiber with a bend ofthe smallest value of radii of curvature R_(th) given by the followingexpression:$R_{th} = {\frac{1}{\{ {{erf}^{- 1}(0.9999)} \}^{2}}( \frac{2\; \pi}{\kappa_{nm}} )^{2}\frac{\pi \; D_{nm}\beta_{m}0.001}{19.09373\; L_{F}}}$where D_(nm) is an intercentral distance between core n and core m inthe multi-core fiber, β_(m) a propagation constant of core m, κ_(nm) acoupling coefficient from core n to core m, and L_(F) a fiber length ofthe multi-core fiber corresponding to a length in laying the opticalfiber cable, said expression defining the radii of curvature with aprobability of 99.99% that crosstalk after propagation through the fiberlength L_(F) is not more than −30 dB, for all combinations of two coresselected from the plurality of cores in the multi-core fiber.
 18. Theoptical fiber cable according to claim 17, wherein on a cross sectionperpendicular to the predetermined axis, each of the plurality of coresin the multi-core fiber has a refractive-index profile of an identicalstructure.
 19. The optical fiber cable according to claim 18, whereinthe intercentral distance between the cores in the multi-core fiber isnot less than 40 μm on the cross section perpendicular to thepredetermined axis, and a relative refractive-index difference Δ of eachcore to the cladding region is not less than 0.37%.
 20. The opticalfiber cable according to claim 17, wherein an arrangement of each of theplurality of cores in the multi-core fiber varies along a longitudinaldirection thereof on the basis of a bending radius direction of the bendprovided for the multi-core fiber, by provision of an elastic twist or aplastic twist.
 21. The optical fiber cable according to claim 20,wherein the multi-core fiber is provided with the twist of not less than2π (rad/m).